diff --git a/library/core/src/main/java/com/google/android/exoplayer2/offline/ProgressiveDownloader.java b/library/core/src/main/java/com/google/android/exoplayer2/offline/ProgressiveDownloader.java
index 17f4047bc0..a73258272c 100644
--- a/library/core/src/main/java/com/google/android/exoplayer2/offline/ProgressiveDownloader.java
+++ b/library/core/src/main/java/com/google/android/exoplayer2/offline/ProgressiveDownloader.java
@@ -29,6 +29,12 @@ import java.util.concurrent.atomic.AtomicBoolean;
 
 /**
  * A downloader for progressive media streams.
+ *
+ * <p>The downloader attempts to download the entire media bytes referenced by a {@link Uri} into a
+ * cache as defined by {@link DownloaderConstructorHelper}. Callers can use the constructor to
+ * specify a custom cache key for the downloaded bytes.
+ *
+ * <p>The downloader will avoid downloading already-downloaded media bytes.
  */
 public final class ProgressiveDownloader implements Downloader {