diff --git a/pw_protobuf/codegen_message_test.cc b/pw_protobuf/codegen_message_test.cc
index 7f1741f..4fee12c 100644
--- a/pw_protobuf/codegen_message_test.cc
+++ b/pw_protobuf/codegen_message_test.cc
@@ -1051,13 +1051,13 @@
 TEST(CodegenMessage, ReadOptionalPresent) {
   // clang-format off
   constexpr uint8_t proto_data[] = {
-    // optional.always_present_fixed
-    0x0d, 0x2a, 0x00, 0x00, 0x00,
-    // optional.always_present_varint
-    0x10, 0x2a,
     // optional.sometimes_present_fixed
-    0x1d, 0x45, 0x00, 0x00, 0x00,
+    0x0d, 0x2a, 0x00, 0x00, 0x00,
     // optional.sometimes_present_varint
+    0x10, 0x2a,
+    // optional.explicitly_present_fixed
+    0x1d, 0x45, 0x00, 0x00, 0x00,
+    // optional.explicitly_present_varint
     0x20, 0x45,
     // optional.sometimes_empty_fixed
     0x2a, 0x04, 0x63, 0x00, 0x00, 0x00,
@@ -1073,12 +1073,12 @@
   const auto status = optional_test.Read(message);
   ASSERT_EQ(status, OkStatus());
 
-  EXPECT_EQ(message.always_present_fixed, 0x2a);
-  EXPECT_EQ(message.always_present_varint, 0x2a);
-  EXPECT_TRUE(message.sometimes_present_fixed);
-  EXPECT_EQ(*message.sometimes_present_fixed, 0x45);
-  EXPECT_TRUE(message.sometimes_present_varint);
-  EXPECT_EQ(*message.sometimes_present_varint, 0x45);
+  EXPECT_EQ(message.sometimes_present_fixed, 0x2a);
+  EXPECT_EQ(message.sometimes_present_varint, 0x2a);
+  EXPECT_TRUE(message.explicitly_present_fixed);
+  EXPECT_EQ(*message.explicitly_present_fixed, 0x45);
+  EXPECT_TRUE(message.explicitly_present_varint);
+  EXPECT_EQ(*message.explicitly_present_varint, 0x45);
   EXPECT_FALSE(message.sometimes_empty_fixed.empty());
   EXPECT_EQ(message.sometimes_empty_fixed.size(), 1u);
   EXPECT_EQ(message.sometimes_empty_fixed[0], 0x63);
@@ -1097,12 +1097,60 @@
   const auto status = optional_test.Read(message);
   ASSERT_EQ(status, OkStatus());
 
-  EXPECT_EQ(message.always_present_fixed, 0);
-  EXPECT_EQ(message.always_present_varint, 0);
-  EXPECT_FALSE(message.sometimes_present_fixed);
-  EXPECT_FALSE(message.sometimes_present_varint);
+  // Non-optional fields have their default value.
+  EXPECT_EQ(message.sometimes_present_fixed, 0);
+  EXPECT_EQ(message.sometimes_present_varint, 0);
   EXPECT_TRUE(message.sometimes_empty_fixed.empty());
   EXPECT_TRUE(message.sometimes_empty_varint.empty());
+
+  // Optional fields are explicitly not present.
+  EXPECT_FALSE(message.explicitly_present_fixed);
+  EXPECT_FALSE(message.explicitly_present_varint);
+}
+
+TEST(CodegenMessage, ReadOptionalPresentDefaults) {
+  // clang-format off
+  constexpr uint8_t proto_data[] = {
+    // optional.sometimes_present_fixed
+    0x0d, 0x00, 0x00, 0x00, 0x00,
+    // optional.sometimes_present_varint
+    0x10, 0x00,
+    // optional.explicitly_present_fixed
+    0x1d, 0x00, 0x00, 0x00, 0x00,
+    // optional.explicitly_present_varint
+    0x20, 0x00,
+    // optional.sometimes_empty_fixed
+    0x2a, 0x04, 0x00, 0x00, 0x00, 0x00,
+    // optional.sometimes_empty_varint
+    0x32, 0x01, 0x00,
+  };
+  // clang-format on
+
+  stream::MemoryReader reader(std::as_bytes(std::span(proto_data)));
+  OptionalTest::StreamDecoder optional_test(reader);
+
+  OptionalTest::Message message{};
+  const auto status = optional_test.Read(message);
+  ASSERT_EQ(status, OkStatus());
+
+  // Non-optional fields have their default value and aren't meaningfully
+  // different from missing.
+  EXPECT_EQ(message.sometimes_present_fixed, 0x00);
+  EXPECT_EQ(message.sometimes_present_varint, 0x00);
+
+  // Optional fields are explicitly present with a default value.
+  EXPECT_TRUE(message.explicitly_present_fixed);
+  EXPECT_EQ(*message.explicitly_present_fixed, 0x00);
+  EXPECT_TRUE(message.explicitly_present_varint);
+  EXPECT_EQ(*message.explicitly_present_varint, 0x00);
+
+  // Repeated fields with a default value are meaningfully non-empty.
+  EXPECT_FALSE(message.sometimes_empty_fixed.empty());
+  EXPECT_EQ(message.sometimes_empty_fixed.size(), 1u);
+  EXPECT_EQ(message.sometimes_empty_fixed[0], 0x00);
+  EXPECT_FALSE(message.sometimes_empty_varint.empty());
+  EXPECT_EQ(message.sometimes_empty_varint.size(), 1u);
+  EXPECT_EQ(message.sometimes_empty_varint[0], 0x00);
 }
 
 class BreakableDecoder : public KeyValuePair::StreamDecoder {
@@ -1200,21 +1248,15 @@
     // pigweed.bin
     0x40, 0x01,
     // pigweed.proto
-    0x4a, 0x1b,
-    // pigweed.proto.bin
-    0x10, 0x00,
-    // pigweed.proto.pigweed_pigweed_bin
-    0x18, 0x00,
+    0x4a, 0x15,
     // pigweed.proto.pigweed_protobuf_bin
     0x20, 0x01,
     // pigweed.proto.meta
-    0x2a, 0x13,
+    0x2a, 0x11,
     // pigweed.proto.meta.file_name
     0x0a, 0x0b, '/', 'e', 't', 'c', '/', 'p', 'a', 's', 's', 'w', 'd',
     // pigweed.proto.meta.status
     0x10, 0x02,
-    // pigweed.proto.meta.protobuf_bin
-    0x18, 0x00,
     // pigweed.proto.meta.pigweed_bin
     0x20, 0x01,
     // pigweed.bytes
@@ -1244,40 +1286,14 @@
 
   // clang-format off
   constexpr uint8_t expected_proto[] = {
-    // pigweed.magic_number (default)
-    0x08, 0x00,
-    // pigweed.ziggy (default)
-    0x10, 0x00,
-    // pigweed.cycles (default)
-    0x19, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
-    // pigweed.ratio (default)
-    0x25, 0x00, 0x00, 0x00, 0x00,
-    // pigweed.pigweed (default)
-    0x3a, 0x02,
-    // pigweed.pigweed.status (default)
-    0x08, 0x00,
-    // pigweed.bin (default)
-    0x40, 0x00,
-    // pigweed.proto (default)
-    0x4a, 0x0e,
-    // pigweed.proto.bin (default)
-    0x10, 0x00,
-    // pigweed.proto.pigweed_pigweed_bin (default)
-    0x18, 0x00,
-    // pigweed.proto.pigweed_protobuf_bin (default)
-    0x20, 0x00,
-    // pigweed.proto.meta (default)
-    0x2a, 0x06,
-    // pigweed.proto.meta.status (default)
-    0x10, 0x00,
-    // pigweed.proto.meta.protobuf_bin (default)
-    0x18, 0x00,
-    // pigweed.proto.meta.pigweed_bin (default)
-    0x20, 0x00,
+    // pigweed.pigweed (empty)
+    0x3a, 0x00,
+    // pigweed.proto
+    0x4a, 0x02,
+    // pigweed.proto.meta (empty)
+    0x2a, 0x00,
     // pigweed.bytes (default)
     0x5a, 0x08, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
-    // pigweed.bungle (default)
-    0x70, 0x00,
   };
   // clang-format on
 
@@ -1460,36 +1476,12 @@
 
   // clang-format off
   constexpr uint8_t expected_proto[] = {
-    // pigweed.magic_number (default)
-    0x08, 0x00,
-    // pigweed.ziggy (default)
-    0x10, 0x00,
-    // pigweed.cycles (default)
-    0x19, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
-    // pigweed.ratio (default)
-    0x25, 0x00, 0x00, 0x00, 0x00,
-    // pigweed.pigweed (default)
-    0x3a, 0x02,
-    // pigweed.pigweed.status (default)
-    0x08, 0x00,
-    // pigweed.bin (default)
-    0x40, 0x00,
+    // pigweed.pigweed (empty)
+    0x3a, 0x00,
     // pigweed.proto (default)
-    0x4a, 0x0e,
-    // pigweed.proto.bin (default)
-    0x10, 0x00,
-    // pigweed.proto.pigweed_pigweed_bin (default)
-    0x18, 0x00,
-    // pigweed.proto.pigweed_protobuf_bin (default)
-    0x20, 0x00,
-    // pigweed.proto.meta (default)
-    0x2a, 0x06,
-    // pigweed.proto.meta.status (default)
-    0x10, 0x00,
-    // pigweed.proto.meta.protobuf_bin (default)
-    0x18, 0x00,
-    // pigweed.proto.meta.pigweed_bin (default)
-    0x20, 0x00,
+    0x4a, 0x02,
+    // pigweed.proto.meta (empty)
+    0x2a, 0x00,
     // pigweed.bytes (default)
     0x5a, 0x08, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
     // pigweed.description
@@ -1500,8 +1492,6 @@
     'r', ' ', 'a', 's', ' ', 'w', 'e', ' ', 'l', 'i', 'k', 'e', ' ', 't', 'o',
     ' ', 'c', 'a', 'l', 'l', ' ', 't', 'h', 'e', 'm', ',', ' ', 'm', 'o', 'd',
     'u', 'l', 'e', 's',
-    // pigweed.bungle (default)
-    0x70, 0x00,
   };
   // clang-format on
 
@@ -1530,42 +1520,16 @@
 
   // clang-format off
   constexpr uint8_t expected_proto[] = {
-    // pigweed.magic_number (default)
-    0x08, 0x00,
-    // pigweed.ziggy (default)
-    0x10, 0x00,
-    // pigweed.cycles (default)
-    0x19, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
-    // pigweed.ratio (default)
-    0x25, 0x00, 0x00, 0x00, 0x00,
-    // pigweed.pigweed (default)
-    0x3a, 0x02,
-    // pigweed.pigweed.status (default)
-    0x08, 0x00,
-    // pigweed.bin (default)
-    0x40, 0x00,
+    // pigweed.pigweed (empty)
+    0x3a, 0x00,
     // pigweed.proto (default)
-    0x4a, 0x0e,
-    // pigweed.proto.bin (default)
-    0x10, 0x00,
-    // pigweed.proto.pigweed_pigweed_bin (default)
-    0x18, 0x00,
-    // pigweed.proto.pigweed_protobuf_bin (default)
-    0x20, 0x00,
-    // pigweed.proto.meta (default)
-    0x2a, 0x06,
-    // pigweed.proto.meta.status (default)
-    0x10, 0x00,
-    // pigweed.proto.meta.protobuf_bin (default)
-    0x18, 0x00,
-    // pigweed.proto.meta.pigweed_bin (default)
-    0x20, 0x00,
+    0x4a, 0x02,
+    // pigweed.proto.meta (empty)
+    0x2a, 0x00,
     // pigweed.bytes (default)
     0x5a, 0x08, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
     // pigweed.special_property
     0x68, 0x2a,
-    // pigweed.bungle (default)
-    0x70, 0x00,
   };
   // clang-format on
 
@@ -1592,17 +1556,13 @@
   // clang-format off
   constexpr uint8_t expected_proto[] = {
     // period.start
-    0x0a, 0x08,
+    0x0a, 0x06,
     // period.start.seconds v=1517949900
     0x08, 0xcc, 0xa7, 0xe8, 0xd3, 0x05,
-    // period.start.nanoseconds v=0 (default)
-    0x10, 0x00,
     // period.end
-    0x12, 0x08,
+    0x12, 0x06,
     // period.end.seconds, v=1517950378
     0x08, 0xaa, 0xab, 0xe8, 0xd3, 0x05,
-    // period.end.nanoseconds, v=0 (default)
-    0x10, 0x00,
   };
   // clang-format on
 
@@ -1709,60 +1669,32 @@
 
   // clang-format off
   constexpr uint8_t expected_proto[] = {
-    // pigweed.magic_number (default)
-    0x08, 0x00,
-    // pigweed.ziggy (default)
-    0x10, 0x00,
-    // pigweed.cycles (default)
-    0x19, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
-    // pigweed.ratio (default)
-    0x25, 0x00, 0x00, 0x00, 0x00,
     // pigweed.device_info
-    0x32, 0x32,
+    0x32, 0x30,
     // pigweed.device_info.device_name
     0x0a, 0x05, 'p', 'i', 'x', 'e', 'l',
     // pigweed.device_info.device_id
     0x15, 0x08, 0x08, 0x08, 0x08,
-    // pigweed.device_info.status
-    0x18, 0x00,
-    // pigweed.proto.nested_pigweed.device_info.attributes[0]
+    // pigweed.device_info.attributes[0]
     0x22, 0x10,
-    // pigweed.proto.nested_pigweed.device_info.attributes[0].key
+    // pigweed.device_info.attributes[0].key
     0x0a, 0x07, 'v', 'e', 'r', 's', 'i', 'o', 'n',
-    // pigweed.proto.nested_pigweed.device_info.attributes[0].value
+    // pigweed.device_info.attributes[0].value
     0x12, 0x05, '5', '.', '3', '.', '1',
-    // pigweed.proto.nested_pigweed.device_info.attributes[1]
+    // pigweed.device_info.attributes[1]
     0x22, 0x10,
-    // pigweed.proto.nested_pigweed.device_info.attributes[1].key
+    // pigweed.device_info.attributes[1].key
     0x0a, 0x04, 'c', 'h', 'i', 'p',
-    // pigweed.proto.nested_pigweed.device_info.attributes[1].value
+    // pigweed.device_info.attributes[1].value
     0x12, 0x08, 'l', 'e', 'f', 't', '-', 's', 'o', 'c',
-    // pigweed.pigweed (default)
-    0x3a, 0x02,
-    // pigweed.pigweed.status (default)
-    0x08, 0x00,
-    // pigweed.bin (default)
-    0x40, 0x00,
+    // pigweed.pigweed (empty)
+    0x3a, 0x00,
     // pigweed.proto (default)
-    0x4a, 0x0e,
-    // pigweed.proto.bin (default)
-    0x10, 0x00,
-    // pigweed.proto.pigweed_pigweed_bin (default)
-    0x18, 0x00,
-    // pigweed.proto.pigweed_protobuf_bin (default)
-    0x20, 0x00,
-    // pigweed.proto.meta (default)
-    0x2a, 0x06,
-    // pigweed.proto.meta.status (default)
-    0x10, 0x00,
-    // pigweed.proto.meta.protobuf_bin (default)
-    0x18, 0x00,
-    // pigweed.proto.meta.pigweed_bin (default)
-    0x20, 0x00,
+    0x4a, 0x02,
+    // pigweed.proto.meta (empty)
+    0x2a, 0x00,
     // pigweed.bytes (default)
     0x5a, 0x08, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
-    // pigweed.bungle (default)
-    0x70, 0x00,
   };
   // clang-format on
 
@@ -1789,10 +1721,10 @@
 
 TEST(CodegenMessage, WriteOptionalPresent) {
   OptionalTest::Message message{};
-  message.always_present_fixed = 0x2a;
-  message.always_present_varint = 0x2a;
-  message.sometimes_present_fixed = 0x45;
-  message.sometimes_present_varint = 0x45;
+  message.sometimes_present_fixed = 0x2a;
+  message.sometimes_present_varint = 0x2a;
+  message.explicitly_present_fixed = 0x45;
+  message.explicitly_present_varint = 0x45;
   message.sometimes_empty_fixed.push_back(0x63);
   message.sometimes_empty_varint.push_back(0x63);
 
@@ -1806,13 +1738,13 @@
 
   // clang-format off
   constexpr uint8_t expected_proto[] = {
-    // optional.always_present_fixed
-    0x0d, 0x2a, 0x00, 0x00, 0x00,
-    // optional.always_present_varint
-    0x10, 0x2a,
     // optional.sometimes_present_fixed
-    0x1d, 0x45, 0x00, 0x00, 0x00,
+    0x0d, 0x2a, 0x00, 0x00, 0x00,
     // optional.sometimes_present_varint
+    0x10, 0x2a,
+    // optional.explicitly_present_fixed
+    0x1d, 0x45, 0x00, 0x00, 0x00,
+    // optional.explicitly_present_varint
     0x20, 0x45,
     // optional.sometimes_empty_fixed
     0x2a, 0x04, 0x63, 0x00, 0x00, 0x00,
@@ -1840,10 +1772,46 @@
 
   // clang-format off
   constexpr uint8_t expected_proto[] = {
-    // optional.always_present_fixed
-    0x0d, 0x00, 0x00, 0x00, 0x00,
-    // optional.always_present_varint
-    0x10, 0x00,
+  };
+  // clang-format on
+
+  ConstByteSpan result = writer.WrittenData();
+  EXPECT_EQ(result.size(), sizeof(expected_proto));
+  EXPECT_EQ(std::memcmp(result.data(), expected_proto, sizeof(expected_proto)),
+            0);
+}
+
+TEST(CodegenMessage, WriteOptionalPresentDefaults) {
+  OptionalTest::Message message{};
+  // Non-optional fields with a default value are not explicitly encoded, so
+  // aren't meaningfully different from one that's just ommitted.
+  message.sometimes_present_fixed = 0x00;
+  message.sometimes_present_varint = 0x00;
+  // Optional fields, even with a default value, are explicitly encoded.
+  message.explicitly_present_fixed = 0x00;
+  message.explicitly_present_varint = 0x00;
+  // Repeated fields with a default value are meaningfully non-empty.
+  message.sometimes_empty_fixed.push_back(0x00);
+  message.sometimes_empty_varint.push_back(0x00);
+
+  std::byte encode_buffer[512];
+
+  stream::MemoryWriter writer(encode_buffer);
+  OptionalTest::StreamEncoder optional_test(writer, ByteSpan());
+
+  const auto status = optional_test.Write(message);
+  ASSERT_EQ(status, OkStatus());
+
+  // clang-format off
+  constexpr uint8_t expected_proto[] = {
+    // optional.explicitly_present_fixed
+    0x1d, 0x00, 0x00, 0x00, 0x00,
+    // optional.explicitly_present_varint
+    0x20, 0x00,
+    // optional.sometimes_empty_fixed
+    0x2a, 0x04, 0x00, 0x00, 0x00, 0x00,
+    // optional.sometimes_empty_varint
+    0x32, 0x01, 0x00,
   };
   // clang-format on
 
diff --git a/pw_protobuf/encoder.cc b/pw_protobuf/encoder.cc
index e6109dc..7d5db02 100644
--- a/pw_protobuf/encoder.cc
+++ b/pw_protobuf/encoder.cc
@@ -14,6 +14,7 @@
 
 #include "pw_protobuf/encoder.h"
 
+#include <algorithm>
 #include <cstddef>
 #include <cstring>
 #include <optional>
@@ -321,7 +322,11 @@
         } else {
           PW_CHECK(values.size() == field.elem_size(),
                    "Mismatched message field type and size");
-          PW_TRY(WriteFixed(field.field_number(), values));
+          if (static_cast<size_t>(
+                  std::count(values.begin(), values.end(), std::byte{0})) <
+              values.size()) {
+            PW_TRY(WriteFixed(field.field_number(), values));
+          }
         }
         break;
       }
@@ -471,6 +476,9 @@
             } else {
               value = *reinterpret_cast<const uint64_t*>(values.data());
             }
+            if (!value) {
+              continue;
+            }
           } else if (field.elem_size() == sizeof(uint32_t)) {
             if (field.varint_type() == VarintType::kZigZag) {
               value = varint::ZigZagEncode(
@@ -480,8 +488,14 @@
             } else {
               value = *reinterpret_cast<const uint32_t*>(values.data());
             }
+            if (!value) {
+              continue;
+            }
           } else if (field.elem_size() == sizeof(bool)) {
             value = *reinterpret_cast<const bool*>(values.data());
+            if (!value) {
+              continue;
+            }
           }
           PW_TRY(WriteVarintField(field.field_number(), value));
         }
diff --git a/pw_protobuf/pw_protobuf_test_protos/optional.proto b/pw_protobuf/pw_protobuf_test_protos/optional.proto
index c17df5f..b75cd17 100644
--- a/pw_protobuf/pw_protobuf_test_protos/optional.proto
+++ b/pw_protobuf/pw_protobuf_test_protos/optional.proto
@@ -16,10 +16,10 @@
 package pw.protobuf.test;
 
 message OptionalTest {
-  sfixed32 always_present_fixed = 1;
-  int32 always_present_varint = 2;
-  optional sfixed32 sometimes_present_fixed = 3;
-  optional int32 sometimes_present_varint = 4;
+  sfixed32 sometimes_present_fixed = 1;
+  int32 sometimes_present_varint = 2;
+  optional sfixed32 explicitly_present_fixed = 3;
+  optional int32 explicitly_present_varint = 4;
   repeated sfixed32 sometimes_empty_fixed = 5;
   repeated int32 sometimes_empty_varint = 6;
 };
diff --git a/pw_rpc/pwpb/serde_test.cc b/pw_rpc/pwpb/serde_test.cc
index b9ae4aa..c5348e6 100644
--- a/pw_rpc/pwpb/serde_test.cc
+++ b/pw_rpc/pwpb/serde_test.cc
@@ -30,12 +30,9 @@
 
   StatusWithSize result = kTestRequest.Encode(kProto, buffer);
   EXPECT_EQ(OkStatus(), result.status());
-  EXPECT_EQ(result.size(), 4u);
+  EXPECT_EQ(result.size(), 2u);
   EXPECT_EQ(buffer[0], std::byte{1} << 3);
   EXPECT_EQ(buffer[1], std::byte{3});
-  // pw_protobuf encodes zero fields
-  EXPECT_EQ(buffer[2], std::byte{2} << 3);
-  EXPECT_EQ(buffer[3], std::byte{0});
 }
 
 TEST(PwpbSerde, Encode_TooSmall) {