diff --git a/pw_stream/memory_stream.cc b/pw_stream/memory_stream.cc
new file mode 100644
index 0000000..efe2c97
--- /dev/null
+++ b/pw_stream/memory_stream.cc
@@ -0,0 +1,36 @@
+// Copyright 2020 The Pigweed Authors
+//
+// 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
+//
+//     https://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.
+
+#include "pw_stream/memory_stream.h"
+
+#include <cstddef>
+#include <cstring>
+
+#include "pw_status/status_with_size.h"
+
+namespace pw::stream {
+
+Status MemoryWriter::DoWrite(span<const std::byte> data) {
+  size_t bytes_to_write =
+      std::min(data.size_bytes(), dest_.size_bytes() - bytes_written_);
+  std::memcpy(dest_.data() + bytes_written_, data.data(), bytes_to_write);
+  bytes_written_ += bytes_to_write;
+
+  if (bytes_to_write != data.size_bytes()) {
+    return Status::RESOURCE_EXHAUSTED;
+  }
+  return Status::OK;
+}
+
+}  // namespace pw::stream
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