diff --git a/build.sbt b/build.sbt
index c1d28b5f..947cc5a6 100644
--- a/build.sbt
+++ b/build.sbt
@@ -6,10 +6,10 @@ name := "facia-api-client"
 
 description := "Scala client for The Guardian's Facia JSON API"
 
-ThisBuild / scalaVersion := "2.13.14"
+ThisBuild / scalaVersion := "2.13.15"
 
 val sonatypeReleaseSettings = Seq(
-  releaseVersion := fromAggregatedAssessedCompatibilityWithLatestRelease().value,
+  // releaseVersion := fromAggregatedAssessedCompatibilityWithLatestRelease().value,
   releaseCrossBuild := true, // true if you cross-build the project for multiple Scala versions
   releaseProcess := Seq[ReleaseStep](
     checkSnapshotDependencies,
@@ -37,7 +37,13 @@ def artifactProducingSettings(supportScala3: Boolean) = Seq(
   libraryDependencies += scalaTest
 )
 
+lazy val fapiClient_core = (project in file("fapi-client-core")).settings(
+  libraryDependencies += eTagCachingS3Base,
+  artifactProducingSettings(supportScala3 = true)
+)
+
 lazy val root = (project in file(".")).aggregate(
+    fapiClient_core,
     faciaJson_play28,
     faciaJson_play29,
     faciaJson_play30,
@@ -55,9 +61,10 @@ def playJsonSpecificProject(module: String, playJsonVersion: PlayJsonVersion) =
   )
 
 def faciaJson(playJsonVersion: PlayJsonVersion) = playJsonSpecificProject("facia-json", playJsonVersion)
+  .dependsOn(fapiClient_core)
   .settings(
     libraryDependencies ++= Seq(
-      awsSdk,
+      awsS3SdkV1, // ideally, this would be pushed out to a separate FAPI artifact
       commonsIo,
       playJsonVersion.lib,
       "org.scala-lang.modules" %% "scala-collection-compat" % "2.11.0",
@@ -69,6 +76,7 @@ def faciaJson(playJsonVersion: PlayJsonVersion) = playJsonSpecificProject("facia
 def fapiClient(playJsonVersion: PlayJsonVersion) =  playJsonSpecificProject("fapi-client", playJsonVersion)
   .settings(
     libraryDependencies ++= Seq(
+      eTagCachingS3SdkV2,
       contentApi,
       contentApiDefault,
       commercialShared,
diff --git a/facia-json/src/main/scala/com/gu/facia/client/S3Client.scala b/facia-json/src/main/scala/com/gu/facia/client/AmazonSdkS3Client.scala
similarity index 80%
rename from facia-json/src/main/scala/com/gu/facia/client/S3Client.scala
rename to facia-json/src/main/scala/com/gu/facia/client/AmazonSdkS3Client.scala
index 1c198454..6f67b033 100644
--- a/facia-json/src/main/scala/com/gu/facia/client/S3Client.scala
+++ b/facia-json/src/main/scala/com/gu/facia/client/AmazonSdkS3Client.scala
@@ -1,18 +1,13 @@
 package com.gu.facia.client
 
-import com.amazonaws.regions.{Region, Regions}
-import com.amazonaws.services.s3.{AmazonS3, AmazonS3ClientBuilder}
+import com.amazonaws.regions.Regions
 import com.amazonaws.services.s3.model.AmazonS3Exception
+import com.amazonaws.services.s3.{AmazonS3, AmazonS3ClientBuilder}
 import org.apache.commons.io.IOUtils
+
 import scala.concurrent.{ExecutionContext, Future, blocking}
 import scala.util.Try
 
-/** For mocking in tests, but also to allow someone to define a properly asynchronous S3 client. (The one in the AWS
-  * SDK is unfortunately synchronous only.)
-  */
-trait S3Client {
-  def get(bucket: String, path: String): Future[FaciaResult]
-}
 
 case class AmazonSdkS3Client(client: AmazonS3)(implicit executionContext: ExecutionContext) extends S3Client {
   def get(bucket: String, path: String): Future[FaciaResult] = Future {
diff --git a/facia-json/src/main/scala/com/gu/facia/client/ApiClient.scala b/facia-json/src/main/scala/com/gu/facia/client/ApiClient.scala
index 5c7936fe..51aba1b8 100644
--- a/facia-json/src/main/scala/com/gu/facia/client/ApiClient.scala
+++ b/facia-json/src/main/scala/com/gu/facia/client/ApiClient.scala
@@ -1,37 +1,82 @@
 package com.gu.facia.client
 
-import com.gu.facia.client.models.{ConfigJson, CollectionJson}
+import com.gu.etagcaching.FreshnessPolicy.AlwaysWaitForRefreshedValue
+import com.gu.etagcaching.aws.s3.{ObjectId, S3ByteArrayFetching}
+import com.gu.etagcaching.fetching.Fetching
+import com.gu.etagcaching.{ConfigCache, ETagCache}
+import com.gu.facia.client.models.{CollectionJson, ConfigJson}
 import play.api.libs.json.{Format, Json}
 
 import scala.concurrent.{ExecutionContext, Future}
 
-object ApiClient {
-  val Encoding = "utf-8"
+trait ApiClient {
+  def config: Future[ConfigJson]
+
+  def collection(id: String): Future[Option[CollectionJson]]
 }
 
-case class ApiClient(
+object ApiClient {
+  private val Encoding = "utf-8"
+
+  /**
+   * Legacy constructor for creating a client that does not support caching. Use `ApiClient.withCaching()` instead.
+   */
+  def apply(
     bucket: String,
-    /** e.g., CODE, PROD, DEV ... */
-    environment: String,
+    environment: String, // e.g., CODE, PROD, DEV ...
     s3Client: S3Client
-)(implicit executionContext: ExecutionContext) {
-  import com.gu.facia.client.ApiClient._
-
-  private def retrieve[A: Format](key: String): Future[Option[A]] = s3Client.get(bucket, key) map {
-    case FaciaSuccess(bytes) =>
-        Some(Json.fromJson[A](Json.parse(new String(bytes, Encoding))) getOrElse {
-          throw new JsonDeserialisationError(s"Could not deserialize JSON in $bucket, $key")
-        })
-    case FaciaNotAuthorized(message) => throw new BackendError(message)
-    case FaciaNotFound(_)  => None
-    case FaciaUnknownError(message)  => throw new BackendError(message)
+  )(implicit executionContext: ExecutionContext): ApiClient = new ApiClient {
+    val env: Environment = Environment(environment)
+
+    private def retrieve[A: Format](key: String): Future[Option[A]] = s3Client.get(bucket, key).map(translateFaciaResult[A](_))
+
+    def config: Future[ConfigJson] =
+      retrieve[ConfigJson](env.configS3Path).map(getOrWarnAboutMissingConfig)
+
+    def collection(id: String): Future[Option[CollectionJson]] =
+      retrieve[CollectionJson](env.collectionS3Path(id))
+  }
+
+  private def getOrWarnAboutMissingConfig(configOpt: Option[ConfigJson]): ConfigJson =
+    configOpt.getOrElse(throw BackendError("Config was missing!! NOT GOOD!"))
+
+  private def translateFaciaResult[B: Format](faciaResult: FaciaResult): Option[B] = faciaResult match {
+    case FaciaSuccess(bytes) => Some(parseBytes(bytes))
+    case FaciaNotAuthorized(message) => throw BackendError(message)
+    case FaciaNotFound(_) => None
+    case FaciaUnknownError(message) => throw BackendError(message)
   }
 
-  def config: Future[ConfigJson] =
-    retrieve[ConfigJson](s"$environment/frontsapi/config/config.json").map(_ getOrElse {
-      throw new BackendError("Config was missing!! OH MY GOD")
-    })
+  private def parseBytes[B: Format](bytes: Array[Byte]): B =
+    Json.fromJson[B](Json.parse(new String(bytes, Encoding))) getOrElse {
+      throw JsonDeserialisationError(s"Could not deserialize JSON")
+    }
+
+  /**
+   * @param s3Fetching see scaladoc on `S3ByteArrayFetching` i.e. use `S3ObjectFetching.byteArraysWith(s3AsyncClient)`
+   */
+  def withCaching(
+    bucket: String,
+    environment: Environment,
+    s3Fetching: S3ByteArrayFetching,
+    configureCollectionCache: ConfigCache = _.maximumSize(10000) // at most 1GB RAM worst case
+  )(implicit ec: ExecutionContext): ApiClient = new ApiClient {
+    private val fetching =
+      s3Fetching.keyOn[String](path => ObjectId(bucket, path))
+
+    def eTagCache[B: Format](configureCache: ConfigCache) = new ETagCache(
+      fetching.thenParsing(parseBytes[B]),
+      AlwaysWaitForRefreshedValue,
+      configureCache
+    )
+
+    private val configCache = eTagCache[ConfigJson](_.maximumSize(1))
+    private val collectionCache = eTagCache[CollectionJson](configureCollectionCache)
 
-  def collection(id: String): Future[Option[CollectionJson]] =
-    retrieve[CollectionJson](s"$environment/frontsapi/collection/$id/collection.json")
+    override def config: Future[ConfigJson] =
+      configCache.get(environment.configS3Path).map(getOrWarnAboutMissingConfig)
+
+    override def collection(id: String): Future[Option[CollectionJson]] =
+      collectionCache.get(environment.collectionS3Path(id))
+  }
 }
diff --git a/facia-json/src/test/scala/com/gu/facia/client/ApiClientSpec.scala b/facia-json/src/test/scala/com/gu/facia/client/ApiClientSpec.scala
index 91f499a2..38ddcf48 100644
--- a/facia-json/src/test/scala/com/gu/facia/client/ApiClientSpec.scala
+++ b/facia-json/src/test/scala/com/gu/facia/client/ApiClientSpec.scala
@@ -1,33 +1,58 @@
 package com.gu.facia.client
 
+import com.gu.etagcaching.aws.s3.ObjectId
+import com.gu.etagcaching.fetching.{ETaggedData, Fetching, Missing, MissingOrETagged}
 import com.gu.facia.client.lib.ResourcesHelper
 import org.scalatest.OptionValues
 import org.scalatest.concurrent.{IntegrationPatience, ScalaFutures}
 import org.scalatest.flatspec.AnyFlatSpec
 import org.scalatest.matchers.should.Matchers
 
-import scala.concurrent.ExecutionContext.Implicits.global
-import scala.concurrent.Future
+import scala.concurrent.{ExecutionContext, Future}
+import scala.util.hashing.MurmurHash3
+
+object FakeS3Fetching extends Fetching[ObjectId, Array[Byte]] with ResourcesHelper {
+  private def pretendETagFor(bytes: Array[Byte]): String = MurmurHash3.bytesHash(bytes).toHexString
+
+  override def fetch(objectId: ObjectId)(implicit ec: ExecutionContext): Future[MissingOrETagged[Array[Byte]]] = Future {
+    slurpBytes(objectId.key).fold(Missing: MissingOrETagged[Array[Byte]]) { bytes =>
+      ETaggedData(pretendETagFor(bytes), bytes)
+    }
+  }
+
+  override def fetchOnlyIfETagChanged(objectId: ObjectId, oldETag: String)(implicit ec: ExecutionContext): Future[Option[MissingOrETagged[Array[Byte]]]] = {
+    fetch(objectId).map {
+      case taggedData: ETaggedData[_] =>
+        Option.unless(oldETag == taggedData.eTag)(taggedData) // simulate a Not-Modified response, if there's no change in ETag
+      case x => Some(x)
+    }
+  }
+}
 
 class ApiClientSpec extends AnyFlatSpec with Matchers with OptionValues with ScalaFutures with IntegrationPatience {
+  import scala.concurrent.ExecutionContext.Implicits.global
+
   object FakeS3Client extends S3Client with ResourcesHelper {
     override def get(bucket: String, path: String): Future[FaciaResult] = Future {
       slurpOrDie(path)
     }
   }
 
-  val client: ApiClient = ApiClient("not used", "DEV", FakeS3Client)
+  val legacyClient: ApiClient = ApiClient("not used", "DEV", FakeS3Client)
+  val cachingClient: ApiClient = ApiClient.withCaching("not used", Environment.Dev, FakeS3Fetching)
 
-  "ApiClient" should "fetch the config" in {
-    val config = client.config.futureValue
+  for ((name, client) <- Map("legacy" -> legacyClient, "caching" -> cachingClient)) {
+    s"$name ApiClient" should "fetch the config" in {
+      val config = client.config.futureValue
 
-    config.collections should have size 334
-    config.fronts should have size 79
-  }
+      config.collections should have size 334
+      config.fronts should have size 79
+    }
 
-  it should "fetch a collection" in {
-    val collectionOpt = client.collection("2409-31b3-83df0-de5a").futureValue
+    it should "fetch a collection" in {
+      val collectionOpt = cachingClient.collection("2409-31b3-83df0-de5a").futureValue
 
-    collectionOpt.value.live should have size 8
+      collectionOpt.value.live should have size 8
+    }
   }
 }
\ No newline at end of file
diff --git a/facia-json/src/test/scala/com/gu/facia/client/lib/ApiTestClient.scala b/facia-json/src/test/scala/com/gu/facia/client/lib/ApiTestClient.scala
index 1fda7b27..5d68c362 100644
--- a/facia-json/src/test/scala/com/gu/facia/client/lib/ApiTestClient.scala
+++ b/facia-json/src/test/scala/com/gu/facia/client/lib/ApiTestClient.scala
@@ -1,27 +1,21 @@
 package com.gu.facia.client.lib
 
-import com.amazonaws.auth.{ AWSStaticCredentialsProvider, BasicAWSCredentials}
-import com.amazonaws.regions.Regions
-import com.amazonaws.services.s3.{AmazonS3, AmazonS3ClientBuilder}
 import com.gu.facia.client.models.{CollectionJson, ConfigJson}
-import com.gu.facia.client.{AmazonSdkS3Client, ApiClient}
+import com.gu.facia.client.{ApiClient, Environment}
 import play.api.libs.json.{Format, Json}
 
 import scala.concurrent.ExecutionContext.Implicits.global
 import scala.concurrent.Future
 
-object Amazon {
-  val amazonS3Client = AmazonS3ClientBuilder.standard().withRegion(Regions.EU_WEST_1).withCredentials(new AWSStaticCredentialsProvider(new BasicAWSCredentials("key", "pass"))).build()
-}
-
-class ApiTestClient extends ApiClient("bucket", "DEV", AmazonSdkS3Client(Amazon.amazonS3Client)) with ResourcesHelper {
+class ApiTestClient extends ApiClient with ResourcesHelper {
+  private val environment = Environment.Dev
 
   private def retrieve[A: Format](key: String): Future[Option[A]] =
     Future.successful(slurp(key).map(Json.parse).flatMap(_.asOpt[A]))
 
   override def config: Future[ConfigJson] =
-    retrieve[ConfigJson](s"$environment/frontsapi/config/config.json").map(_.get)
+    retrieve[ConfigJson](environment.configS3Path).map(_.get)
 
   override def collection(id: String): Future[Option[CollectionJson]] =
-    retrieve[CollectionJson](s"$environment/frontsapi/collection/$id/collection.json")
+    retrieve[CollectionJson](environment.collectionS3Path(id))
 }
diff --git a/facia-json/src/test/scala/com/gu/facia/client/lib/ResourcesHelper.scala b/facia-json/src/test/scala/com/gu/facia/client/lib/ResourcesHelper.scala
index 4a0f1e34..c862aed3 100644
--- a/facia-json/src/test/scala/com/gu/facia/client/lib/ResourcesHelper.scala
+++ b/facia-json/src/test/scala/com/gu/facia/client/lib/ResourcesHelper.scala
@@ -1,11 +1,17 @@
 package com.gu.facia.client.lib
 
 import com.gu.facia.client.FaciaSuccess
+import org.apache.commons.io.IOUtils
+
+import java.nio.charset.StandardCharsets.UTF_8
 
 trait ResourcesHelper {
+  def slurpBytes(path: String): Option[Array[Byte]] =
+    Option(getClass.getClassLoader.getResource(path)).map(url => IOUtils.toByteArray(url.openStream()))
+
   def slurp(path: String): Option[String] =
-    Option(getClass.getClassLoader.getResource(path)).map(scala.io.Source.fromURL(_).mkString)
+    slurpBytes(path).map(bytes => new String(bytes, UTF_8))
 
-  def slurpOrDie(path: String) = slurp(path).map(_.getBytes).map(FaciaSuccess.apply).getOrElse {
+  def slurpOrDie(path: String) = slurpBytes(path).map(FaciaSuccess.apply).getOrElse {
     throw new RuntimeException(s"Required resource $path not on class path")  }
 }
\ No newline at end of file
diff --git a/fapi-client-core/src/main/scala/com/gu/facia/client/Environment.scala b/fapi-client-core/src/main/scala/com/gu/facia/client/Environment.scala
new file mode 100644
index 00000000..8df3ce0f
--- /dev/null
+++ b/fapi-client-core/src/main/scala/com/gu/facia/client/Environment.scala
@@ -0,0 +1,15 @@
+package com.gu.facia.client
+
+case class Environment(name: String) {
+  private val s3PathPrefix: String = s"$name/frontsapi"
+
+  val configS3Path: String = s"$s3PathPrefix/config/config.json"
+  def collectionS3Path(id: String): String = s"$s3PathPrefix/collection/$id/collection.json"
+}
+
+object Environment {
+  val Prod = Environment("PROD")
+  val Code = Environment("CODE")
+  val Dev = Environment("DEV")
+  val Test = Environment("TEST")
+}
\ No newline at end of file
diff --git a/facia-json/src/main/scala/com/gu/facia/client/FaciaResult.scala b/fapi-client-core/src/main/scala/com/gu/facia/client/FaciaResult.scala
similarity index 100%
rename from facia-json/src/main/scala/com/gu/facia/client/FaciaResult.scala
rename to fapi-client-core/src/main/scala/com/gu/facia/client/FaciaResult.scala
diff --git a/facia-json/src/main/scala/com/gu/facia/client/JsonDeserialisationError.scala b/fapi-client-core/src/main/scala/com/gu/facia/client/JsonDeserialisationError.scala
similarity index 100%
rename from facia-json/src/main/scala/com/gu/facia/client/JsonDeserialisationError.scala
rename to fapi-client-core/src/main/scala/com/gu/facia/client/JsonDeserialisationError.scala
diff --git a/fapi-client-core/src/main/scala/com/gu/facia/client/S3Client.scala b/fapi-client-core/src/main/scala/com/gu/facia/client/S3Client.scala
new file mode 100644
index 00000000..ed741f1c
--- /dev/null
+++ b/fapi-client-core/src/main/scala/com/gu/facia/client/S3Client.scala
@@ -0,0 +1,16 @@
+package com.gu.facia.client
+
+import scala.concurrent.Future
+
+/**
+ * Legacy class for mocking in tests, but also previously used to allow library users to define a properly
+ * asynchronous S3 client (back when the one in the AWS SDK was synchronous only).
+ *
+ * Note that use of `S3Client` is now discouraged, as `facia-scala-client` now supports caching using the
+ * `etag-caching` library, which provides its own more powerful abstraction for fetching & parsing data, and
+ * `com.gu.etagcaching.aws.sdkv2.s3.S3ObjectFetching`, a ready-made implementation of fetching that includes
+ * support for ETags & Conditional Requests, decoding S3 exceptions, etc.
+ */
+trait S3Client {
+  def get(bucket: String, path: String): Future[FaciaResult]
+}
diff --git a/fapi-client-core/src/main/scala/com/gu/facia/client/package.scala b/fapi-client-core/src/main/scala/com/gu/facia/client/package.scala
new file mode 100644
index 00000000..5924c031
--- /dev/null
+++ b/fapi-client-core/src/main/scala/com/gu/facia/client/package.scala
@@ -0,0 +1,16 @@
+package com.gu.facia
+
+import com.gu.etagcaching.aws.s3.ObjectId
+import com.gu.etagcaching.fetching.Fetching
+
+package object client {
+  /**
+   * AWS SDK v2
+   *
+   * Add "com.gu.etag-caching" %% "aws-s3-sdk-v2" as dependency,
+   * import com.gu.etagcaching.aws.sdkv2.s3.S3ObjectFetching
+   *
+   * S3ObjectFetching.byteArrayWith(s3AsyncClient)
+   */
+  type S3FetchBehaviour = Fetching[ObjectId, Array[Byte]]
+}
diff --git a/fapi-client/src/test/scala/lib/IntegrationTestConfig.scala b/fapi-client/src/test/scala/lib/IntegrationTestConfig.scala
index 81697e67..d9cdd418 100644
--- a/fapi-client/src/test/scala/lib/IntegrationTestConfig.scala
+++ b/fapi-client/src/test/scala/lib/IntegrationTestConfig.scala
@@ -1,11 +1,16 @@
 package lib
 
-import com.amazonaws.auth._
-import com.amazonaws.auth.profile.ProfileCredentialsProvider
-import com.amazonaws.services.s3.AmazonS3ClientBuilder
+//import com.amazonaws.auth._
+import software.amazon.awssdk.auth.credentials.{EnvironmentVariableCredentialsProvider, ProfileCredentialsProvider}
+// import com.amazonaws.auth.profile.ProfileCredentialsProvider
+//import com.amazonaws.services.s3.AmazonS3ClientBuilder
 import com.gu.contentapi.client.GuardianContentClient
-import com.gu.facia.client.{AmazonSdkS3Client, ApiClient}
-import com.amazonaws.regions.Regions.EU_WEST_1
+import com.gu.etagcaching.aws.sdkv2.s3.S3ObjectFetching
+import com.gu.etagcaching.aws.sdkv2.s3.response.Transformer.Bytes
+import com.gu.facia.client.{ApiClient, Environment}
+import software.amazon.awssdk.auth.credentials.{AwsCredentialsProvider, AwsCredentialsProviderChain}
+import software.amazon.awssdk.regions.Region
+import software.amazon.awssdk.services.s3.S3AsyncClient
 
 private case class TestContentApiClient(override val apiKey: String, override val targetUrl: String)
   extends GuardianContentClient(apiKey)
@@ -22,16 +27,16 @@ trait IntegrationTestConfig extends ExecutionContext {
     }
   }
 
-  implicit val apiClient: ApiClient = {
-    val credentialsProvider = new AWSCredentialsProviderChain(
-      new EnvironmentVariableCredentialsProvider(),
-      new SystemPropertiesCredentialsProvider(),
-      new ProfileCredentialsProvider(awsProfileName)
-    )
-    val amazonS3Client = AmazonS3ClientBuilder.standard()
-      .withRegion(EU_WEST_1)
-      .withCredentials(credentialsProvider)
-      .build()
-    ApiClient("facia-tool-store", "DEV", AmazonSdkS3Client(amazonS3Client))
-  }
+  def credentialsForDevAndCI(devProfile: String, ciCreds: AwsCredentialsProvider): AwsCredentialsProviderChain =
+    AwsCredentialsProviderChain.of(ciCreds, ProfileCredentialsProvider.builder().profileName(devProfile).build())
+
+  private val s3AsyncClient: S3AsyncClient = S3AsyncClient.builder()
+    .region(Region.EU_WEST_1)
+    .credentialsProvider(credentialsForDevAndCI(awsProfileName, EnvironmentVariableCredentialsProvider.create())).build()
+
+  implicit val apiClient: ApiClient = ApiClient.withCaching(
+    "facia-tool-store",
+    Environment.Dev,
+    S3ObjectFetching.byteArraysWith(s3AsyncClient)
+  )
 }
diff --git a/project/dependencies.scala b/project/dependencies.scala
index 71171aa1..9b10cd17 100644
--- a/project/dependencies.scala
+++ b/project/dependencies.scala
@@ -2,8 +2,11 @@ import sbt._
 
 object Dependencies {
   val capiVersion = "32.0.0"
+  val eTagCachingVersion = "6.0.0-PREVIEW.add-convenience-for-fetching-s3-byte-arrays.2024-12-13T1224.a635ab54"
 
-  val awsSdk = "com.amazonaws" % "aws-java-sdk-s3" % "1.12.673"
+  val awsS3SdkV1 = "com.amazonaws" % "aws-java-sdk-s3" % "1.12.765"
+  val eTagCachingS3Base = "com.gu.etag-caching" %% "aws-s3-base" % eTagCachingVersion
+  val eTagCachingS3SdkV2 = "com.gu.etag-caching" %% "aws-s3-sdk-v2" % eTagCachingVersion
   val commonsIo = "org.apache.commons" % "commons-io" % "1.3.2"
   val contentApi = "com.gu" %% "content-api-client" % capiVersion
   val contentApiDefault = "com.gu" %% "content-api-client-default" % capiVersion % Test