diff --git a/adapters/src/main/java/com/squareup/moshi/adapters/RuntimeJsonAdapterFactory.java b/adapters/src/main/java/com/squareup/moshi/adapters/RuntimeJsonAdapterFactory.java
index 7ff47b1..0cb5f69 100644
--- a/adapters/src/main/java/com/squareup/moshi/adapters/RuntimeJsonAdapterFactory.java
+++ b/adapters/src/main/java/com/squareup/moshi/adapters/RuntimeJsonAdapterFactory.java
@@ -103,7 +103,7 @@ final class RuntimeJsonAdapterFactory<T> implements JsonAdapter.Factory {
       return null;
     }
 
-    List<JsonAdapter<Object>> jsonAdapters = new ArrayList<>();
+    List<JsonAdapter<Object>> jsonAdapters = new ArrayList<>(subtypes.size());
     for (int i = 0, size = subtypes.size(); i < size; i++) {
       jsonAdapters.add(moshi.adapter(subtypes.get(i)));
     }
diff --git a/moshi/src/main/java/com/squareup/moshi/JsonUtf8Writer.java b/moshi/src/main/java/com/squareup/moshi/JsonUtf8Writer.java
index 7a7a727..5aa749a 100644
--- a/moshi/src/main/java/com/squareup/moshi/JsonUtf8Writer.java
+++ b/moshi/src/main/java/com/squareup/moshi/JsonUtf8Writer.java
@@ -83,7 +83,7 @@ final class JsonUtf8Writer extends JsonWriter {
           "Array cannot be used as a map key in JSON at path " + getPath());
     }
     writeDeferredName();
-    return open(EMPTY_ARRAY, "[");
+    return open(EMPTY_ARRAY, NONEMPTY_ARRAY, "[");
   }
 
   @Override public JsonWriter endArray() throws IOException {
@@ -96,7 +96,7 @@ final class JsonUtf8Writer extends JsonWriter {
           "Object cannot be used as a map key in JSON at path " + getPath());
     }
     writeDeferredName();
-    return open(EMPTY_OBJECT, "{");
+    return open(EMPTY_OBJECT, NONEMPTY_OBJECT, "{");
   }
 
   @Override public JsonWriter endObject() throws IOException {
@@ -108,7 +108,13 @@ final class JsonUtf8Writer extends JsonWriter {
    * Enters a new scope by appending any necessary whitespace and the given
    * bracket.
    */
-  private JsonWriter open(int empty, String openBracket) throws IOException {
+  private JsonWriter open(int empty, int nonempty, String openBracket) throws IOException {
+    if (stackSize == flattenStackSize
+        && (scopes[stackSize - 1] == empty || scopes[stackSize - 1] == nonempty)) {
+      // Cancel this open. Invert the flatten stack size until this is closed.
+      flattenStackSize = ~flattenStackSize;
+      return this;
+    }
     beforeValue();
     checkStack();
     pushScope(empty);
@@ -129,6 +135,11 @@ final class JsonUtf8Writer extends JsonWriter {
     if (deferredName != null) {
       throw new IllegalStateException("Dangling name: " + deferredName);
     }
+    if (stackSize == ~flattenStackSize) {
+      // Cancel this close. Restore the flattenStackSize so we're ready to flatten again!
+      flattenStackSize = ~flattenStackSize;
+      return this;
+    }
 
     stackSize--;
     pathNames[stackSize] = null; // Free the last path name so that it can be garbage collected!
diff --git a/moshi/src/main/java/com/squareup/moshi/JsonValueWriter.java b/moshi/src/main/java/com/squareup/moshi/JsonValueWriter.java
index 64878ef..6ab79ee 100644
--- a/moshi/src/main/java/com/squareup/moshi/JsonValueWriter.java
+++ b/moshi/src/main/java/com/squareup/moshi/JsonValueWriter.java
@@ -52,6 +52,11 @@ final class JsonValueWriter extends JsonWriter {
       throw new IllegalStateException(
           "Array cannot be used as a map key in JSON at path " + getPath());
     }
+    if (stackSize == flattenStackSize && scopes[stackSize - 1] == EMPTY_ARRAY) {
+      // Cancel this open. Invert the flatten stack size until this is closed.
+      flattenStackSize = ~flattenStackSize;
+      return this;
+    }
     checkStack();
     List<Object> list = new ArrayList<>();
     add(list);
@@ -65,6 +70,11 @@ final class JsonValueWriter extends JsonWriter {
     if (peekScope() != EMPTY_ARRAY) {
       throw new IllegalStateException("Nesting problem.");
     }
+    if (stackSize == ~flattenStackSize) {
+      // Cancel this close. Restore the flattenStackSize so we're ready to flatten again!
+      flattenStackSize = ~flattenStackSize;
+      return this;
+    }
     stackSize--;
     stack[stackSize] = null;
     pathIndices[stackSize - 1]++;
@@ -76,6 +86,11 @@ final class JsonValueWriter extends JsonWriter {
       throw new IllegalStateException(
           "Object cannot be used as a map key in JSON at path " + getPath());
     }
+    if (stackSize == flattenStackSize && scopes[stackSize - 1] == EMPTY_OBJECT) {
+      // Cancel this open. Invert the flatten stack size until this is closed.
+      flattenStackSize = ~flattenStackSize;
+      return this;
+    }
     checkStack();
     Map<String, Object> map = new LinkedHashTreeMap<>();
     add(map);
@@ -91,6 +106,11 @@ final class JsonValueWriter extends JsonWriter {
     if (deferredName != null) {
       throw new IllegalStateException("Dangling name: " + deferredName);
     }
+    if (stackSize == ~flattenStackSize) {
+      // Cancel this close. Restore the flattenStackSize so we're ready to flatten again!
+      flattenStackSize = ~flattenStackSize;
+      return this;
+    }
     promoteValueToName = false;
     stackSize--;
     stack[stackSize] = null;
diff --git a/moshi/src/main/java/com/squareup/moshi/JsonWriter.java b/moshi/src/main/java/com/squareup/moshi/JsonWriter.java
index 678171c..fa4137b 100644
--- a/moshi/src/main/java/com/squareup/moshi/JsonWriter.java
+++ b/moshi/src/main/java/com/squareup/moshi/JsonWriter.java
@@ -24,7 +24,9 @@ import javax.annotation.Nullable;
 import okio.BufferedSink;
 import okio.BufferedSource;
 
+import static com.squareup.moshi.JsonScope.EMPTY_ARRAY;
 import static com.squareup.moshi.JsonScope.EMPTY_OBJECT;
+import static com.squareup.moshi.JsonScope.NONEMPTY_ARRAY;
 import static com.squareup.moshi.JsonScope.NONEMPTY_OBJECT;
 
 /**
@@ -140,6 +142,26 @@ public abstract class JsonWriter implements Closeable, Flushable {
   boolean serializeNulls;
   boolean promoteValueToName;
 
+  /**
+   * Controls the deepest stack size that has begin/end pairs flattened:
+   *
+   * <ul>
+   *     <li>If -1, no begin/end pairs are being suppressed.
+   *     <li>If positive, this is the deepest stack size whose begin/end pairs are eligible to be
+   *         flattened.
+   *     <li>If negative, it is the bitwise inverse (~) of the deepest stack size whose begin/end
+   *         pairs have been flattened.
+   * </ul>
+   *
+   * <p>We differentiate between what layer would be flattened (positive) from what layer is being
+   * flattened (negative) so that we don't double-flatten.
+   *
+   * <p>To accommodate nested flattening we require callers to track the previous state when they
+   * provide a new state. The previous state is returned from {@link #beginFlatten} and restored
+   * with {@link #endFlatten}.
+   */
+  int flattenStackSize = -1;
+
   /** Returns a new instance that writes UTF-8 encoded JSON to {@code sink}. */
   @CheckReturnValue public static JsonWriter of(BufferedSink sink) {
     return new JsonUtf8Writer(sink);
@@ -357,6 +379,88 @@ public abstract class JsonWriter implements Closeable, Flushable {
     promoteValueToName = true;
   }
 
+  /**
+   * Cancels immediately-nested calls to {@link #beginArray()} or {@link #beginObject()} and their
+   * matching calls to {@link #endArray} or {@link #endObject()}. Use this to compose JSON adapters
+   * without nesting.
+   *
+   * <p>For example, the following creates JSON with nested arrays: {@code [1,[2,3,4],5]}.
+   *
+   * <pre>{@code
+   *
+   *   JsonAdapter<List<Integer>> integersAdapter = ...
+   *
+   *   public void writeNumbers(JsonWriter writer) {
+   *     writer.beginArray();
+   *     writer.value(1);
+   *     integersAdapter.toJson(writer, Arrays.asList(2, 3, 4));
+   *     writer.value(5);
+   *     writer.endArray();
+   *   }
+   * }</pre>
+   *
+   * <p>With flattening we can create JSON with a single array {@code [1,2,3,4,5]}:
+   *
+   * <pre>{@code
+   *
+   *   JsonAdapter<List<Integer>> integersAdapter = ...
+   *
+   *   public void writeNumbers(JsonWriter writer) {
+   *     writer.beginArray();
+   *     int token = writer.beginFlatten();
+   *     writer.value(1);
+   *     integersAdapter.toJson(writer, Arrays.asList(2, 3, 4));
+   *     writer.value(5);
+   *     writer.endFlatten(token);
+   *     writer.endArray();
+   *   }
+   * }</pre>
+   *
+   * <p>This method flattens arrays within arrays:
+   *
+   * <pre>{@code
+   *
+   *   Emit:       [1, [2, 3, 4], 5]
+   *   To produce: [1, 2, 3, 4, 5]
+   * }</pre>
+   *
+   * It also flattens objects within objects. Do not call {@link #name} before writing a flattened
+   * object.
+   *
+   * <pre>{@code
+   *
+   *   Emit:       {"a": 1, {"b": 2}, "c": 3}
+   *   To Produce: {"a": 1, "b": 2, "c": 3}
+   * }</pre>
+   *
+   * Other combinations are permitted but do not perform flattening. For example, objects inside of
+   * arrays are not flattened:
+   *
+   * <pre>{@code
+   *
+   *   Emit:       [1, {"b": 2}, 3, [4, 5], 6]
+   *   To Produce: [1, {"b": 2}, 3, 4, 5, 6]
+   * }</pre>
+   *
+   * <p>This method returns an opaque token. Callers must match all calls to this method with a call
+   * to {@link #endFlatten} with the matching token.
+   */
+  public final int beginFlatten() {
+    int context = peekScope();
+    if (context != NONEMPTY_OBJECT && context != EMPTY_OBJECT
+        && context != NONEMPTY_ARRAY && context != EMPTY_ARRAY) {
+      throw new IllegalStateException("Nesting problem.");
+    }
+    int token = flattenStackSize;
+    flattenStackSize = stackSize;
+    return token;
+  }
+
+  /** Ends nested call flattening created by {@link #beginFlatten}. */
+  public final void endFlatten(int token) {
+    flattenStackSize = token;
+  }
+
   /**
    * Returns a <a href="http://goessner.net/articles/JsonPath/">JsonPath</a> to
    * the current location in the JSON value.
diff --git a/moshi/src/test/java/com/squareup/moshi/FlattenTest.java b/moshi/src/test/java/com/squareup/moshi/FlattenTest.java
new file mode 100644
index 0000000..d69e2fa
--- /dev/null
+++ b/moshi/src/test/java/com/squareup/moshi/FlattenTest.java
@@ -0,0 +1,338 @@
+/*
+ * Copyright (C) 2018 Square, Inc.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package com.squareup.moshi;
+
+import java.util.Arrays;
+import java.util.List;
+import org.junit.Test;
+import org.junit.runner.RunWith;
+import org.junit.runners.Parameterized;
+import org.junit.runners.Parameterized.Parameter;
+import org.junit.runners.Parameterized.Parameters;
+
+import static org.assertj.core.api.Assertions.assertThat;
+import static org.junit.Assert.fail;
+
+/** Note that this test makes heavy use of nested blocks, but these are for readability only. */
+@RunWith(Parameterized.class)
+public final class FlattenTest {
+  @Parameter public JsonCodecFactory factory;
+
+  @Parameters(name = "{0}")
+  public static List<Object[]> parameters() {
+    return JsonCodecFactory.factories();
+  }
+
+  @Test public void flattenExample() throws Exception {
+    Moshi moshi = new Moshi.Builder().build();
+    JsonAdapter<List<Integer>> integersAdapter =
+        moshi.adapter(Types.newParameterizedType(List.class, Integer.class));
+
+    JsonWriter writer = factory.newWriter();
+    writer.beginArray();
+    int token = writer.beginFlatten();
+    writer.value(1);
+    integersAdapter.toJson(writer, Arrays.asList(2, 3, 4));
+    writer.value(5);
+    writer.endFlatten(token);
+    writer.endArray();
+
+    assertThat(factory.json()).isEqualTo("[1,2,3,4,5]");
+  }
+
+  @Test public void flattenObject() throws Exception {
+    JsonWriter writer = factory.newWriter();
+    writer.beginObject();
+    {
+      writer.name("a");
+      writer.value("aaa");
+      int token = writer.beginFlatten();
+      {
+        writer.beginObject();
+        {
+          writer.name("b");
+          writer.value("bbb");
+        }
+        writer.endObject();
+      }
+      writer.endFlatten(token);
+      writer.name("c");
+      writer.value("ccc");
+    }
+    writer.endObject();
+    assertThat(factory.json()).isEqualTo("{\"a\":\"aaa\",\"b\":\"bbb\",\"c\":\"ccc\"}");
+  }
+
+  @Test public void flattenArray() throws Exception {
+    JsonWriter writer = factory.newWriter();
+    writer.beginArray();
+    {
+      writer.value("a");
+      int token = writer.beginFlatten();
+      {
+        writer.beginArray();
+        {
+          writer.value("b");
+        }
+        writer.endArray();
+      }
+      writer.endFlatten(token);
+      writer.value("c");
+    }
+    writer.endArray();
+    assertThat(factory.json()).isEqualTo("[\"a\",\"b\",\"c\"]");
+  }
+
+  @Test public void recursiveFlatten() throws Exception {
+    JsonWriter writer = factory.newWriter();
+    writer.beginArray();
+    {
+      writer.value("a");
+      int token1 = writer.beginFlatten();
+      {
+        writer.beginArray();
+        {
+          writer.value("b");
+          int token2 = writer.beginFlatten();
+          {
+            writer.beginArray();
+            {
+              writer.value("c");
+            }
+            writer.endArray();
+          }
+          writer.endFlatten(token2);
+          writer.value("d");
+        }
+        writer.endArray();
+      }
+      writer.endFlatten(token1);
+      writer.value("e");
+    }
+    writer.endArray();
+    assertThat(factory.json()).isEqualTo("[\"a\",\"b\",\"c\",\"d\",\"e\"]");
+  }
+
+  @Test public void flattenMultipleNested() throws Exception {
+    JsonWriter writer = factory.newWriter();
+    writer.beginArray();
+    {
+      writer.value("a");
+      int token = writer.beginFlatten();
+      {
+        writer.beginArray();
+        {
+          writer.value("b");
+        }
+        writer.endArray();
+        writer.beginArray();
+        {
+          writer.value("c");
+        }
+        writer.endArray();
+      }
+      writer.endFlatten(token);
+      writer.value("d");
+    }
+    writer.endArray();
+    assertThat(factory.json()).isEqualTo("[\"a\",\"b\",\"c\",\"d\"]");
+  }
+
+  @Test public void flattenIsOnlyOneLevelDeep() throws Exception {
+    JsonWriter writer = factory.newWriter();
+    writer.beginArray();
+    {
+      writer.value("a");
+      int token = writer.beginFlatten();
+      {
+        writer.beginArray();
+        {
+          writer.value("b");
+          writer.beginArray();
+          {
+            writer.value("c");
+          }
+          writer.endArray();
+          writer.value("d");
+        }
+        writer.endArray();
+      }
+      writer.endFlatten(token);
+      writer.value("e");
+    }
+    writer.endArray();
+    assertThat(factory.json()).isEqualTo("[\"a\",\"b\",[\"c\"],\"d\",\"e\"]");
+  }
+
+  @Test public void flattenOnlySomeChildren() throws Exception {
+    JsonWriter writer = factory.newWriter();
+    writer.beginArray();
+    {
+      writer.value("a");
+      int token = writer.beginFlatten();
+      {
+        writer.beginArray();
+        {
+          writer.value("b");
+        }
+        writer.endArray();
+      }
+      writer.endFlatten(token);
+      writer.beginArray();
+      {
+        writer.value("c");
+      }
+      writer.endArray();
+      writer.value("d");
+    }
+    writer.endArray();
+    assertThat(factory.json()).isEqualTo("[\"a\",\"b\",[\"c\"],\"d\"]");
+  }
+
+  @Test public void multipleCallsToFlattenSameNesting() throws Exception {
+    JsonWriter writer = factory.newWriter();
+    writer.beginArray();
+    {
+      writer.value("a");
+      int token1 = writer.beginFlatten();
+      {
+        writer.beginArray();
+        {
+          writer.value("b");
+        }
+        writer.endArray();
+        int token2 = writer.beginFlatten();
+        {
+          writer.beginArray();
+          {
+            writer.value("c");
+          }
+          writer.endArray();
+        }
+        writer.endFlatten(token2);
+        writer.beginArray();
+        {
+          writer.value("d");
+        }
+        writer.endArray();
+      }
+      writer.endFlatten(token1);
+      writer.value("e");
+    }
+    writer.endArray();
+    assertThat(factory.json()).isEqualTo("[\"a\",\"b\",\"c\",\"d\",\"e\"]");
+  }
+
+  @Test public void deepFlatten() throws Exception {
+    JsonWriter writer = factory.newWriter();
+    writer.beginArray();
+    {
+      int token1 = writer.beginFlatten();
+      {
+        writer.beginArray();
+        {
+          int token2 = writer.beginFlatten();
+          {
+            writer.beginArray();
+            {
+              int token3 = writer.beginFlatten();
+              {
+                writer.beginArray();
+                {
+                  writer.value("a");
+                }
+                writer.endArray();
+              }
+              writer.endFlatten(token3);
+            }
+            writer.endArray();
+          }
+          writer.endFlatten(token2);
+        }
+        writer.endArray();
+      }
+      writer.endFlatten(token1);
+    }
+    writer.endArray();
+    assertThat(factory.json()).isEqualTo("[\"a\"]");
+  }
+
+  @Test public void flattenTopLevel() {
+    JsonWriter writer = factory.newWriter();
+    try {
+      writer.beginFlatten();
+      fail();
+    } catch (IllegalStateException e) {
+      assertThat(e).hasMessage("Nesting problem.");
+    }
+  }
+
+  @Test public void flattenDoesNotImpactOtherTypesInObjects() throws Exception {
+    JsonWriter writer = factory.newWriter();
+    writer.beginObject();
+    {
+      int token = writer.beginFlatten();
+      writer.name("a");
+      writer.beginArray();
+      writer.value("aaa");
+      writer.endArray();
+      writer.beginObject();
+      {
+        writer.name("b");
+        writer.value("bbb");
+      }
+      writer.endObject();
+      writer.name("c");
+      writer.beginArray();
+      writer.value("ccc");
+      writer.endArray();
+      writer.endFlatten(token);
+    }
+    writer.endObject();
+    assertThat(factory.json()).isEqualTo("{\"a\":[\"aaa\"],\"b\":\"bbb\",\"c\":[\"ccc\"]}");
+  }
+
+  @Test public void flattenDoesNotImpactOtherTypesInArrays() throws Exception {
+    JsonWriter writer = factory.newWriter();
+    writer.beginArray();
+    {
+      int token = writer.beginFlatten();
+      {
+        writer.beginObject();
+        {
+          writer.name("a");
+          writer.value("aaa");
+        }
+        writer.endObject();
+        writer.beginArray();
+        {
+          writer.value("bbb");
+        }
+        writer.endArray();
+        writer.value("ccc");
+        writer.beginObject();
+        {
+          writer.name("d");
+          writer.value("ddd");
+        }
+        writer.endObject();
+      }
+      writer.endFlatten(token);
+    }
+    writer.endArray();
+    assertThat(factory.json()).isEqualTo("[{\"a\":\"aaa\"},\"bbb\",\"ccc\",{\"d\":\"ddd\"}]");
+  }
+}