diff --git a/README.md b/README.md
index b1b5fa1..7fcde09 100644
--- a/README.md
+++ b/README.md
@@ -218,7 +218,7 @@ $ pipenv run tests
 - [x] Repeated message fields
 - [x] Maps
   - [x] Maps of message fields
-- [ ] Support passthrough of unknown fields
+- [x] Support passthrough of unknown fields
 - [x] Refs to nested types
 - [x] Imports in proto files
 - [x] Well-known Google types
diff --git a/betterproto/__init__.py b/betterproto/__init__.py
index e53f869..b0fb445 100644
--- a/betterproto/__init__.py
+++ b/betterproto/__init__.py
@@ -341,17 +341,19 @@ class ParsedField:
     number: int
     wire_type: int
     value: Any
+    raw: bytes
 
 
 def parse_fields(value: bytes) -> Generator[ParsedField, None, None]:
     i = 0
     while i < len(value):
+        start = i
         num_wire, i = decode_varint(value, i)
         # print(num_wire, i)
         number = num_wire >> 3
         wire_type = num_wire & 0x7
 
-        decoded: Any
+        decoded: Any = None
         if wire_type == 0:
             decoded, i = decode_varint(value, i)
         elif wire_type == 1:
@@ -362,12 +364,12 @@ def parse_fields(value: bytes) -> Generator[ParsedField, None, None]:
             i += length
         elif wire_type == 5:
             decoded, i = value[i : i + 4], i + 4
-        else:
-            raise NotImplementedError(f"Wire type {wire_type}")
 
         # print(ParsedField(number=number, wire_type=wire_type, value=decoded))
 
-        yield ParsedField(number=number, wire_type=wire_type, value=decoded)
+        yield ParsedField(
+            number=number, wire_type=wire_type, value=decoded, raw=value[start:i]
+        )
 
 
 # Bound type variable to allow methods to return `self` of subclasses
@@ -415,6 +417,7 @@ class Message(ABC):
 
         # Now that all the defaults are set, reset it!
         self.__dict__["serialized_on_wire"] = False
+        self.__dict__["_unknown_fields"] = b""
 
     def __setattr__(self, attr: str, value: Any) -> None:
         if attr != "serialized_on_wire":
@@ -469,7 +472,7 @@ class Message(ABC):
                     meta.number, meta.proto_type, value, serialize_empty=serialize_empty
                 )
 
-        return output
+        return output + self._unknown_fields
 
     # For compatibility with other libraries
     SerializeToString = __bytes__
@@ -571,8 +574,7 @@ class Message(ABC):
                 else:
                     setattr(self, field.name, value)
             else:
-                # TODO: handle unknown fields
-                pass
+                self._unknown_fields += parsed.raw
 
         return self
 
diff --git a/betterproto/tests/test_features.py b/betterproto/tests/test_features.py
index 4a6c4d4..8afaa74 100644
--- a/betterproto/tests/test_features.py
+++ b/betterproto/tests/test_features.py
@@ -48,3 +48,25 @@ def test_enum_as_int_json():
     # Plain-ol'-ints should serialize properly too.
     foo.bar = 1
     assert foo.to_dict() == {"bar": "ONE"}
+
+
+def test_unknown_fields():
+    @dataclass
+    class Newer(betterproto.Message):
+        foo: bool = betterproto.bool_field(1)
+        bar: int = betterproto.int32_field(2)
+        baz: str = betterproto.string_field(3)
+
+    @dataclass
+    class Older(betterproto.Message):
+        foo: bool = betterproto.bool_field(1)
+
+    newer = Newer(foo=True, bar=1, baz="Hello")
+    serialized_newer = bytes(newer)
+
+    # Unknown fields in `Newer` should round trip with `Older`
+    round_trip = bytes(Older().parse(serialized_newer))
+    assert serialized_newer == round_trip
+
+    new_again = Newer().parse(round_trip)
+    assert newer == new_again