diff --git a/config.json b/config.json
index 745b61777..4f1adc9e7 100644
--- a/config.json
+++ b/config.json
@@ -40,6 +40,7 @@
     "circular-buffer",
     "robot-name",
     "react",
+    "counter",
     "custom-set",
     "atbash-cipher",
     "phone-number",
diff --git a/counter/counter_test.go b/counter/counter_test.go
new file mode 100644
index 000000000..90ebbe5a6
--- /dev/null
+++ b/counter/counter_test.go
@@ -0,0 +1,3 @@
+package counter
+
+// Define your tests here
diff --git a/counter/example.go b/counter/example.go
new file mode 100644
index 000000000..1cba4b9ee
--- /dev/null
+++ b/counter/example.go
@@ -0,0 +1,63 @@
+package counter
+
+// NOTE: This file is called example.go and not example_test.go because if it's
+// called example_test.go it will get picked up by the test suite and fail
+// because COUNTER_IMPL isn't set (see maker.go).
+
+import "testing"
+
+func TestNoAdd(t *testing.T) {
+	counter := makeCounter()
+	if counter.Lines() != 0 {
+		t.Errorf("Lines mismatch: got %d, expected %d", counter.Lines(), 0)
+	}
+	if counter.Letters() != 0 {
+		t.Errorf("Letters mismatch: got %d, expected %d", counter.Letters(), 0)
+	}
+	if counter.Characters() != 0 {
+		t.Errorf("Characters mismatch: got %d, expected %d", counter.Characters(), 0)
+	}
+}
+
+func TestEmptyString(t *testing.T) {
+	counter := makeCounter()
+	counter.AddString("")
+	if counter.Lines() != 0 {
+		t.Errorf("Lines mismatch: got %d, expected %d", counter.Lines(), 0)
+	}
+	if counter.Letters() != 0 {
+		t.Errorf("Letters mismatch: got %d, expected %d", counter.Letters(), 0)
+	}
+	if counter.Characters() != 0 {
+		t.Errorf("Characters mismatch: got %d, expected %d", counter.Characters(), 0)
+	}
+}
+
+func TestASCIIString(t *testing.T) {
+	counter := makeCounter()
+	counter.AddString("Hello\nworld!")
+	if counter.Lines() != 2 {
+		t.Errorf("Lines mismatch: got %d, expected %d", counter.Lines(), 2)
+	}
+	if counter.Letters() != 10 {
+		t.Errorf("Letters mismatch: got %d, expected %d", counter.Letters(), 10)
+	}
+	if counter.Characters() != 12 {
+		t.Errorf("Characters mismatch: got %d, expected %d", counter.Characters(), 12)
+	}
+}
+
+func TestRussianString(t *testing.T) {
+	counter := makeCounter()
+	// Lifted this translation from the ru.po file of GNU hello
+	counter.AddString("здравствуй, мир\n")
+	if counter.Lines() != 1 {
+		t.Errorf("Lines mismatch: got %d, expected %d", counter.Lines(), 2)
+	}
+	if counter.Letters() != 13 {
+		t.Errorf("Letters mismatch: got %d, expected %d", counter.Letters(), 10)
+	}
+	if counter.Characters() != 16 {
+		t.Errorf("Characters mismatch: got %d, expected %d", counter.Characters(), 12)
+	}
+}
diff --git a/counter/impl1.go b/counter/impl1.go
new file mode 100644
index 000000000..e279510b5
--- /dev/null
+++ b/counter/impl1.go
@@ -0,0 +1,23 @@
+package counter
+
+import "unicode"
+
+// Incorrect implementation: wrongly counts lines.
+type Impl1 struct {
+	lines, characters, letters int
+}
+
+func (c *Impl1) AddString(s string) {
+	for _, char := range s {
+		if char == '\n' {
+			c.lines++
+		} else if unicode.IsLetter(char) {
+			c.letters++
+		}
+		c.characters++
+	}
+}
+
+func (c Impl1) Lines() int      { return c.lines }
+func (c Impl1) Letters() int    { return c.letters }
+func (c Impl1) Characters() int { return c.characters }
diff --git a/counter/impl2.go b/counter/impl2.go
new file mode 100644
index 000000000..0ed404153
--- /dev/null
+++ b/counter/impl2.go
@@ -0,0 +1,33 @@
+package counter
+
+// Incorrect implementation: wrongly determines characters.
+type Impl2 struct {
+	newlines, characters, letters int
+	lastChar                      rune
+}
+
+func (c *Impl2) AddString(s string) {
+	for _, char := range s {
+		c.lastChar = char
+		if char == '\n' {
+			c.newlines++
+		} else if (char >= 'A' && char <= 'Z') || (char >= 'a' && char <= 'z') {
+			c.letters++
+		}
+		c.characters++
+	}
+}
+
+func (c Impl2) Lines() int {
+	switch {
+	case c.characters == 0:
+		return 0
+	case c.lastChar == '\n':
+		return c.newlines
+	default:
+		return c.newlines + 1
+	}
+}
+
+func (c Impl2) Letters() int    { return c.letters }
+func (c Impl2) Characters() int { return c.characters }
diff --git a/counter/impl3.go b/counter/impl3.go
new file mode 100644
index 000000000..36c9125bd
--- /dev/null
+++ b/counter/impl3.go
@@ -0,0 +1,36 @@
+package counter
+
+import "unicode"
+
+// Incorrect implementation: assumes ASCII.
+type Impl3 struct {
+	newlines, characters, letters int
+	lastChar                      rune
+}
+
+func (c *Impl3) AddString(s string) {
+	for i := 0; i < len(s); i++ {
+		char := rune(s[i])
+		c.lastChar = char
+		if char == '\n' {
+			c.newlines++
+		} else if unicode.IsLetter(char) {
+			c.letters++
+		}
+		c.characters++
+	}
+}
+
+func (c Impl3) Lines() int {
+	switch {
+	case c.characters == 0:
+		return 0
+	case c.lastChar == '\n':
+		return c.newlines
+	default:
+		return c.newlines + 1
+	}
+}
+
+func (c Impl3) Letters() int    { return c.letters }
+func (c Impl3) Characters() int { return c.characters }
diff --git a/counter/impl4.go b/counter/impl4.go
new file mode 100644
index 000000000..7081eefd3
--- /dev/null
+++ b/counter/impl4.go
@@ -0,0 +1,35 @@
+package counter
+
+import "unicode"
+
+// Correct implementation
+type Impl4 struct {
+	newlines, characters, letters int
+	lastChar                      rune
+}
+
+func (c *Impl4) AddString(s string) {
+	for _, char := range s {
+		c.lastChar = char
+		if char == '\n' {
+			c.newlines++
+		} else if unicode.IsLetter(char) {
+			c.letters++
+		}
+		c.characters++
+	}
+}
+
+func (c Impl4) Lines() int {
+	switch {
+	case c.characters == 0:
+		return 0
+	case c.lastChar == '\n':
+		return c.newlines
+	default:
+		return c.newlines + 1
+	}
+}
+
+func (c Impl4) Letters() int    { return c.letters }
+func (c Impl4) Characters() int { return c.characters }
diff --git a/counter/interface.go b/counter/interface.go
new file mode 100644
index 000000000..5286a9e74
--- /dev/null
+++ b/counter/interface.go
@@ -0,0 +1,9 @@
+package counter
+
+type Counter interface {
+	AddString(string)
+
+	Lines() int
+	Letters() int
+	Characters() int
+}
diff --git a/counter/maker.go b/counter/maker.go
new file mode 100644
index 000000000..674f17dcd
--- /dev/null
+++ b/counter/maker.go
@@ -0,0 +1,29 @@
+package counter
+
+import (
+	"log"
+	"os"
+)
+
+// A little trick to help test the various versions,
+// set the COUNTER_IMPL environment variable to the
+// number of the implementation you want to test.
+// E.g. run `COUNTER_IMPL=4 go test` to get Impl4.
+
+func makeCounter() Counter {
+	switch os.Getenv("COUNTER_IMPL") {
+	case "1":
+		return &Impl1{}
+	case "2":
+		return &Impl2{}
+	case "3":
+		return &Impl3{}
+	case "4":
+		return &Impl4{}
+	case "":
+		log.Fatalf("Don't forget to set COUNTER_IMPL")
+	default:
+		log.Fatalf("Unknown COUNTER_IMPL value: %s", os.Getenv("COUNTER_IMPL"))
+	}
+	panic("not reachable, but go's return analysis needs it")
+}