diff --git a/fgpyo/fasta/builder.py b/fgpyo/fasta/builder.py
index bb58316d..3efdc640 100755
--- a/fgpyo/fasta/builder.py
+++ b/fgpyo/fasta/builder.py
@@ -43,7 +43,6 @@ def samtools_dict(*args: Any) -> None:
     def samtools_faidx(*args: Any) -> None:
         pass
 
-
 else:
     from pysam import dict as samtools_dict
     from pysam import faidx as samtools_faidx
diff --git a/fgpyo/io/__init__.py b/fgpyo/io/__init__.py
index cb47ed99..34c95139 100644
--- a/fgpyo/io/__init__.py
+++ b/fgpyo/io/__init__.py
@@ -128,7 +128,7 @@ def assert_path_is_writeable(path: Path, parent_must_exist: bool = True) -> None
 
 
 def to_reader(path: Path) -> Union[io.TextIOWrapper, TextIO, IO[Any]]:
-    """Opens a Path for reading.
+    """Opens a Path for reading and based on extension uses open() or gzip.open()
 
     Args:
         path: Path to read from
@@ -144,21 +144,27 @@ def to_reader(path: Path) -> Union[io.TextIOWrapper, TextIO, IO[Any]]:
         return path.open(mode="r")
 
 
-def to_writer(path: Path) -> Union[IO[Any], io.TextIOWrapper]:
-    """Opens a Path for reading and based on extension uses open() or gzip.open()
+def to_writer(path: Path, append: bool = False) -> Union[IO[Any], io.TextIOWrapper]:
+    """Opens a Path for writing (or appending) and based on extension uses open() or gzip.open()
 
     Args:
-        path: Path to write to
+        path: Path to write (or append) to
 
     Example:
         writer = fio.to_writer(path = Path("writer.txt"))
         writer.write(f'{something}\n')
         writer.close()
     """
+    mode_prefix = "w"
+    if append:
+        mode_prefix = "a"
+
     if path.suffix in COMPRESSED_FILE_EXTENSIONS:
-        return io.TextIOWrapper(cast(IO[bytes], gzip.open(path, mode="wb")), encoding="utf-8")
+        return io.TextIOWrapper(
+            cast(IO[bytes], gzip.open(path, mode=mode_prefix + "b")), encoding="utf-8"
+        )
     else:
-        return path.open(mode="w")
+        return path.open(mode=mode_prefix)
 
 
 def read_lines(path: Path, strip: bool = False) -> Iterator[str]:
@@ -183,19 +189,19 @@ def read_lines(path: Path, strip: bool = False) -> Iterator[str]:
                 yield line.rstrip("\r\n")
 
 
-def write_lines(path: Path, lines_to_write: Iterable[Any]) -> None:
-    """Writes a file with one line per item in provided iterable
+def write_lines(path: Path, lines_to_write: Iterable[Any], append: bool = False) -> None:
+    """Writes (or appends) a file with one line per item in provided iterable
 
     Args:
-        path: Path to write to
-        lines_to_write: items to write to file
+        path: Path to write (or append) to
+        lines_to_write: items to write (or append) to file
 
     Example:
         lines: List[Any] = ["things to write", 100]
         path_to_write_to: Path = Path("file_to_write_to.txt")
         fio.write_lines(path = path_to_write_to, lines_to_write = lines)
     """
-    with to_writer(path=path) as writer:
+    with to_writer(path=path, append=append) as writer:
         for line in lines_to_write:
             writer.write(str(line))
             writer.write("\n")
diff --git a/fgpyo/util/metric.py b/fgpyo/util/metric.py
index 2bff416f..a4681994 100644
--- a/fgpyo/util/metric.py
+++ b/fgpyo/util/metric.py
@@ -305,3 +305,17 @@ def format_value(cls, value: Any) -> str:
     def to_list(cls, value: str) -> List[Any]:
         """Returns a list value split on comma delimeter."""
         return [] if value == "" else value.split(",")
+
+    @staticmethod
+    def fast_concat(*inputs: Path, output: Path) -> None:
+        if len(inputs) == 0:
+            raise ValueError("No inputs provided")
+
+        headers = [next(io.read_lines(input_path)) for input_path in inputs]
+        assert len(set(headers)) == 1, "Input headers do not match"
+        io.write_lines(path=output, lines_to_write=set(headers))
+
+        for input_path in inputs:
+            io.write_lines(
+                path=output, lines_to_write=list(io.read_lines(input_path))[1:], append=True
+            )
diff --git a/fgpyo/util/tests/test_metric.py b/fgpyo/util/tests/test_metric.py
index d011873c..5f2e2eaa 100644
--- a/fgpyo/util/tests/test_metric.py
+++ b/fgpyo/util/tests/test_metric.py
@@ -319,3 +319,26 @@ def test_metric_list_parse_with_none() -> None:
     assert ListPerson.parse(fields=["Max,Sally", ","]) == ListPerson(
         name=["Max", "Sally"], age=[None, None]
     )
+
+
+def test_metrics_fast_concat(tmpdir: TmpDir) -> None:
+    path_input = []
+    path_input.append(Path(tmpdir) / "metrics_1.txt")
+    path_input.append(Path(tmpdir) / "metrics_2.txt")
+    path_input.append(Path(tmpdir) / "metrics_3.txt")
+    path_output: Path = Path(tmpdir) / "metrics_concat.txt"
+
+    DummyMetric.write(path_input[0], DUMMY_METRICS[0])
+    DummyMetric.write(path_input[1], DUMMY_METRICS[1])
+    DummyMetric.write(path_input[2], DUMMY_METRICS[2])
+
+    Metric.fast_concat(*path_input, output=path_output)
+    metrics: List[DummyMetric] = list(DummyMetric.read(path=path_output))
+
+    assert len(metrics) == len(DUMMY_METRICS)
+    assert metrics[0].header() == DummyMetric.header()
+    assert metrics[1].header() == DummyMetric.header()
+    assert metrics[2].header() == DummyMetric.header()
+    assert metrics[0] == DUMMY_METRICS[0]
+    assert metrics[1] == DUMMY_METRICS[1]
+    assert metrics[2] == DUMMY_METRICS[2]