diff --git a/buffer.c b/buffer.c
index f5d8efe..94819f5 100644
--- a/buffer.c
+++ b/buffer.c
@@ -6,6 +6,7 @@
 
 #include <string.h>
 #include <stdio.h>
+#include <stdarg.h>
 #include <stdlib.h>
 #include <ctype.h>
 #include <sys/types.h>
@@ -139,6 +140,35 @@ buffer_resize(buffer_t *self, size_t n) {
   return 0;
 }
 
+/*
+ * Append a printf-style formatted string to the buffer.
+ */
+int buffer_appendf(buffer_t *self, const char *format, ...) {
+  va_list ap;
+  va_start(ap, format);
+  const int initial_len = buffer_length(self);
+
+  // First, we compute how many bytes are needed for the formatted string
+  // and allocate that much more space in the buffer.
+  va_list tmpa;
+  va_copy(tmpa, ap);
+  const int space_required = vsnprintf(NULL, 0, format, tmpa);
+  va_end(tmpa);
+  const int resized = buffer_resize(self, initial_len + space_required);
+  if (resized == -1) {
+    va_end(ap);
+    return -1;
+  }
+
+  // Next format the string into the space that we have made room for.
+  char* dst = self->data + initial_len;
+  const int bytes_formatted = vsnprintf(dst, 1+space_required, format, ap);
+
+  printf("Result '%s'\n", self->data);
+  va_end(ap);
+  return (bytes_formatted < 0) ? -1 : 0;
+}
+
 /*
  * Append `str` to `self` and return 0 on success, -1 on failure.
  */
diff --git a/buffer.h b/buffer.h
index 5826b56..5cecbb0 100644
--- a/buffer.h
+++ b/buffer.h
@@ -60,6 +60,9 @@ buffer_prepend(buffer_t *self, char *str);
 int
 buffer_append(buffer_t *self, const char *str);
 
+int
+buffer_appendf(buffer_t *self, const char *format, ...);
+
 int
 buffer_append_n(buffer_t *self, const char *str, size_t len);
 
diff --git a/test.c b/test.c
index 0c750dd..cb6299d 100644
--- a/test.c
+++ b/test.c
@@ -148,6 +148,17 @@ test_buffer_equals() {
   buffer_free(b);
 }
 
+void test_buffer_formatting() {
+  buffer_t *buf = buffer_new();
+  int result = buffer_appendf(buf, "%d %s", 3, "cow");
+  assert(0 == result);
+  equal("3 cow", buffer_string(buf));
+  result = buffer_appendf(buf, " - 0x%08X", 0xdeadbeef);
+  assert(0 == result);
+  equal("3 cow - 0xDEADBEEF", buffer_string(buf));
+  buffer_free(buf);
+}
+
 void
 test_buffer_indexof() {
   buffer_t *buf = buffer_new_with_copy("Tobi is a ferret");
@@ -231,6 +242,7 @@ main(){
   test_buffer_slice__end();
   test_buffer_slice__end_overflow();
   test_buffer_equals();
+  test_buffer_formatting();
   test_buffer_indexof();
   test_buffer_fill();
   test_buffer_clear();