diff --git a/src/main/scala/net/kemitix/s3thorp/awssdk/HashLookup.scala b/src/main/scala/net/kemitix/s3thorp/awssdk/HashLookup.scala
new file mode 100644
index 0000000..b6332d9
--- /dev/null
+++ b/src/main/scala/net/kemitix/s3thorp/awssdk/HashLookup.scala
@@ -0,0 +1,11 @@
+package net.kemitix.s3thorp.awssdk
+
+import net.kemitix.s3thorp.Sync.{LastModified, MD5Hash, RemoteKey}
+
+/**
+  * A list of objects and their MD5 hash values.
+  */
+case class HashLookup(byHash: Map[MD5Hash, (RemoteKey, LastModified)],
+                      byKey: Map[RemoteKey, (MD5Hash, LastModified)]) {
+
+}
diff --git a/src/main/scala/net/kemitix/s3thorp/awssdk/ThorpS3Client.scala b/src/main/scala/net/kemitix/s3thorp/awssdk/ThorpS3Client.scala
index 8ba970f..fb14728 100644
--- a/src/main/scala/net/kemitix/s3thorp/awssdk/ThorpS3Client.scala
+++ b/src/main/scala/net/kemitix/s3thorp/awssdk/ThorpS3Client.scala
@@ -4,22 +4,49 @@ import cats.effect.IO
 import com.github.j5ik2o.reactive.aws.s3.cats.S3CatsIOClient
 import net.kemitix.s3thorp.Sync._
 import software.amazon.awssdk.core.async.AsyncRequestBody
-import software.amazon.awssdk.services.s3.model.{HeadObjectRequest, PutObjectRequest}
+import software.amazon.awssdk.services.s3.model.{HeadObjectRequest, ListObjectsV2Request, PutObjectRequest, S3Object}
+
+import scala.collection.JavaConverters._
 
 private class ThorpS3Client(s3Client: S3CatsIOClient) extends S3Client {
 
-  def objectHead(bucket: Bucket, remoteKey: RemoteKey) = {
-    val request = HeadObjectRequest.builder().bucket(bucket).key(remoteKey).build()
+  def objectHead(bucket: Bucket, remoteKey: RemoteKey): IO[Option[(MD5Hash, LastModified)]] = {
+    val request = HeadObjectRequest.builder()
+      .bucket(bucket)
+      .key(remoteKey)
+      .build()
     s3Client.headObject(request).attempt.map {
-      case Right(response) => Some((response.eTag(), response.lastModified()))
+      case Right(r) => Some((r.eTag(), r.lastModified()))
       case Left(_) => None
     }
   }
 
   def upload(localFile: LocalFile, bucket: Bucket, remoteKey: RemoteKey): IO[Either[Throwable, MD5Hash]] = {
-    val request = PutObjectRequest.builder().bucket(bucket).key(remoteKey).build()
+    val request = PutObjectRequest.builder()
+      .bucket(bucket)
+      .key(remoteKey)
+      .build()
     val body = AsyncRequestBody.fromFile(localFile)
-    s3Client.putObject(request, body).map{response => Right(response.eTag())}
+    s3Client.putObject(request, body).map(r => Right(r.eTag()))
+  }
+
+  private def asHashLookup: Stream[S3Object] => HashLookup =
+    os => HashLookup(byHash(os), byKey(os))
+
+  private def byHash(os: Stream[S3Object]) =
+    os.map{o => (o.eTag, (o.key, o.lastModified))}.toMap
+
+  private def byKey(os: Stream[S3Object]) =
+    os.map{o => (o.key(), (o.eTag(), o.lastModified()))}.toMap
+
+  def listObjects(bucket: Bucket, prefix: RemoteKey): IO[HashLookup] = {
+    val request = ListObjectsV2Request.builder()
+      .bucket(bucket)
+      .prefix(prefix)
+      .build()
+    s3Client.listObjectsV2(request)
+      .map(r => r.contents.asScala.toStream)
+      .map(asHashLookup)
   }
 
 }
diff --git a/src/test/scala/net/kemitix/s3thorp/awssdk/ThorpS3ClientSuite.scala b/src/test/scala/net/kemitix/s3thorp/awssdk/ThorpS3ClientSuite.scala
new file mode 100644
index 0000000..5f8a8d3
--- /dev/null
+++ b/src/test/scala/net/kemitix/s3thorp/awssdk/ThorpS3ClientSuite.scala
@@ -0,0 +1,49 @@
+package net.kemitix.s3thorp.awssdk
+
+import java.time.Instant
+import scala.collection.JavaConverters._
+
+import cats.effect.IO
+import com.github.j5ik2o.reactive.aws.s3.S3AsyncClient
+import com.github.j5ik2o.reactive.aws.s3.cats.S3CatsIOClient
+import org.scalatest.FunSpec
+import software.amazon.awssdk.services.s3
+import software.amazon.awssdk.services.s3.model.{ListObjectsV2Request, ListObjectsV2Response, S3Object}
+
+class ThorpS3ClientSuite extends FunSpec {
+
+  describe("listObjectsInPrefix") {
+    val h1 = "hash1"
+    val k1 = "key1"
+    val lm1 = Instant.now
+    val o1 = S3Object.builder.eTag(h1).key(k1).lastModified(lm1).build
+    val h2 = "hash2"
+    val k2 = "key2"
+    val lm2 = Instant.now.minusSeconds(200)
+    val o2 = S3Object.builder.eTag(h2).key(k2).lastModified(lm2).build
+    val myFakeResponse: IO[ListObjectsV2Response] = IO{
+      ListObjectsV2Response.builder()
+        .contents(List(o1, o2).asJava)
+        .build()
+    }
+    val subject = new ThorpS3Client(new S3CatsIOClient {
+      override val underlying: S3AsyncClient = new S3AsyncClient {
+        override val underlying: s3.S3AsyncClient = new s3.S3AsyncClient {
+          override def serviceName(): String = "fake-s3-client"
+
+          override def close(): Unit = ()
+        }
+      }
+      override def listObjectsV2(listObjectsV2Request: ListObjectsV2Request) =
+        myFakeResponse
+    })
+    it("should build list of hash lookups") {
+      val result: HashLookup = subject.listObjects("bucket", "prefix").unsafeRunSync()
+      val expected = HashLookup(
+        Map(h1 -> (k1, lm1), h2 -> (k2, lm2)),
+        Map(k1 -> (h1, lm1), k2 -> (h2, lm2)))
+      assertResult(expected)(result)
+    }
+  }
+
+}