diff --git a/factorial/Makefile b/factorial/Makefile
new file mode 100644
index 0000000..e5c288c
--- /dev/null
+++ b/factorial/Makefile
@@ -0,0 +1,13 @@
+
+TARGET := factorial
+DFLAGS := -funittest
+
+all: $(TARGET)
+
+$(TARGET): $(TARGET).d
+
+%: %.d
+	gdc -o $@ $(DFLAGS) $<
+
+clean:
+	-rm -f *.o *~ $(TARGET)
diff --git a/factorial/factorial.d b/factorial/factorial.d
new file mode 100644
index 0000000..a916c65
--- /dev/null
+++ b/factorial/factorial.d
@@ -0,0 +1,41 @@
+import std.stdio;
+
+uint factorial(int a)
+{
+    /*
+     * A factorial is a mathematical concept that
+     * is typically denoted with a "!".
+     * Example:
+     * 4! = 4 * 3 * 2 * 1 = 24
+     */
+
+    if(a == 0) return 1;
+
+    uint b = 1;
+    for(int i = 1; i <= a; i++)
+        b *= i;
+    return b;
+}
+
+unittest
+{
+    /* Compile with the "-unittest" option to run these unittests. */
+
+    writefln("Attempting unittests...");
+    assert(factorial(0) == 1);
+    assert(factorial(1) == 1);
+    assert(factorial(2) == 2);
+    assert(factorial(3) == 6);
+    assert(factorial(4) == 24);
+    writefln("unittests successful...");
+}
+
+
+void main()
+{
+    writefln("factorial(0): %d", factorial(0));
+    writefln("factorial(1): %d", factorial(1));
+    writefln("factorial(2): %d", factorial(2));
+    writefln("factorial(3): %d", factorial(3));
+    writefln("factorial(4): %d", factorial(4));
+}