diff --git a/spacy/tests/test_align.py b/spacy/tests/test_align.py
new file mode 100644
index 000000000..26f0ea867
--- /dev/null
+++ b/spacy/tests/test_align.py
@@ -0,0 +1,38 @@
+import pytest
+from .._align import align
+
+
+@pytest.mark.parametrize('string1,string2,cost', [
+    (b'hello', b'hell', 1),
+    (b'rat', b'cat', 1),
+    (b'rat', b'rat', 0),
+    (b'rat', b'catsie', 4),
+    (b't', b'catsie', 5),
+])
+def test_align_costs(string1, string2, cost):
+    output_cost, i2j, j2i, matrix = align(string1, string2)
+    assert output_cost == cost
+
+
+@pytest.mark.parametrize('string1,string2,i2j', [
+    (b'hello', b'hell', [0,1,2,3,-1]),
+    (b'rat', b'cat', [0,1,2]),
+    (b'rat', b'rat', [0,1,2]),
+    (b'rat', b'catsie', [0,1,2]),
+    (b't', b'catsie', [2]),
+])
+def test_align_i2j(string1, string2, i2j):
+    output_cost, output_i2j, j2i, matrix = align(string1, string2)
+    assert list(output_i2j) == i2j
+
+
+@pytest.mark.parametrize('string1,string2,j2i', [
+    (b'hello', b'hell', [0,1,2,3]),
+    (b'rat', b'cat', [0,1,2]),
+    (b'rat', b'rat', [0,1,2]),
+    (b'rat', b'catsie', [0,1,2, -1, -1, -1]),
+    (b't', b'catsie', [-1, -1, 0, -1, -1, -1]),
+])
+def test_align_i2j(string1, string2, j2i):
+    output_cost, output_i2j, output_j2i, matrix = align(string1, string2)
+    assert list(output_j2i) == j2i