diff --git a/docs/x509.rst b/docs/x509.rst
index d09651f..19f7c40 100644
--- a/docs/x509.rst
+++ b/docs/x509.rst
@@ -276,6 +276,7 @@
             >>> for ext in cert.extensions:
             ...     print(ext)
             <Extension(oid=<ObjectIdentifier(oid=2.5.29.14, name=subjectKeyIdentifier)>, critical=False, value=<SubjectKeyIdentifier(digest='X\x01\x84$\x1b\xbc+R\x94J=\xa5\x10r\x14Q\xf5\xaf:\xc9')>)>
+            <Extension(oid=<ObjectIdentifier(oid=2.5.29.15, name=keyUsage)>, critical=True, value=<KeyUsage(digital_signature=False, content_commitment=False, key_encipherment=False, data_encipherment=False, key_agreement=False, key_cert_sign=True, crl_sign=True, encipher_only=N/A, decipher_only=N/A)>)>
             <Extension(oid=<ObjectIdentifier(oid=2.5.29.19, name=basicConstraints)>, critical=True, value=<BasicConstraints(ca=True, path_length=None)>)>
 
 X.509 CSR (Certificate Signing Request) Object
diff --git a/src/cryptography/hazmat/backends/openssl/x509.py b/src/cryptography/hazmat/backends/openssl/x509.py
index 5d47c5e..57e6146 100644
--- a/src/cryptography/hazmat/backends/openssl/x509.py
+++ b/src/cryptography/hazmat/backends/openssl/x509.py
@@ -14,7 +14,6 @@
 from __future__ import absolute_import, division, print_function
 
 import datetime
-import warnings
 
 from cryptography import utils, x509
 from cryptography.exceptions import UnsupportedAlgorithm
@@ -172,14 +171,8 @@
                 value = self._build_basic_constraints(ext)
             elif oid == x509.OID_SUBJECT_KEY_IDENTIFIER:
                 value = self._build_subject_key_identifier(ext)
-            elif oid == x509.OID_KEY_USAGE and critical:
-                # TODO: remove this obviously.
-                warnings.warn(
-                    "Extension support is not fully implemented. A key usage "
-                    "extension with the critical flag was seen and IGNORED."
-                )
-                seen_oids.add(oid)
-                continue
+            elif oid == x509.OID_KEY_USAGE:
+                value = self._build_key_usage(ext)
             elif critical:
                 raise x509.UnsupportedExtension(
                     "{0} is not currently supported".format(oid), oid
@@ -232,6 +225,35 @@
             self._backend._ffi.buffer(asn1_string.data, asn1_string.length)[:]
         )
 
+    def _build_key_usage(self, ext):
+        bit_string = self._backend._lib.X509V3_EXT_d2i(ext)
+        assert bit_string != self._backend._ffi.NULL
+        bit_string = self._backend._ffi.cast("ASN1_BIT_STRING *", bit_string)
+        bit_string = self._backend._ffi.gc(
+            bit_string, self._backend._lib.ASN1_BIT_STRING_free
+        )
+        get_bit = self._backend._lib.ASN1_BIT_STRING_get_bit
+        digital_signature = get_bit(bit_string, 0) == 1
+        content_commitment = get_bit(bit_string, 1) == 1
+        key_encipherment = get_bit(bit_string, 2) == 1
+        data_encipherment = get_bit(bit_string, 3) == 1
+        key_agreement = get_bit(bit_string, 4) == 1
+        key_cert_sign = get_bit(bit_string, 5) == 1
+        crl_sign = get_bit(bit_string, 6) == 1
+        encipher_only = get_bit(bit_string, 7) == 1
+        decipher_only = get_bit(bit_string, 8) == 1
+        return x509.KeyUsage(
+            digital_signature,
+            content_commitment,
+            key_encipherment,
+            data_encipherment,
+            key_agreement,
+            key_cert_sign,
+            crl_sign,
+            encipher_only,
+            decipher_only
+        )
+
 
 @utils.register_interface(x509.CertificateSigningRequest)
 class _CertificateSigningRequest(object):
diff --git a/src/cryptography/hazmat/bindings/openssl/asn1.py b/src/cryptography/hazmat/bindings/openssl/asn1.py
index 45dfe75..475bd05 100644
--- a/src/cryptography/hazmat/bindings/openssl/asn1.py
+++ b/src/cryptography/hazmat/bindings/openssl/asn1.py
@@ -120,6 +120,7 @@
 """
 
 MACROS = """
+void ASN1_BIT_STRING_free(ASN1_BIT_STRING *);
 /* This is not a macro, but is const on some versions of OpenSSL */
 int ASN1_BIT_STRING_get_bit(ASN1_BIT_STRING *, int);
 ASN1_TIME *M_ASN1_TIME_dup(void *);
diff --git a/tests/test_x509_ext.py b/tests/test_x509_ext.py
index c2d33d9..acfe761 100644
--- a/tests/test_x509_ext.py
+++ b/tests/test_x509_ext.py
@@ -463,3 +463,63 @@
             cert.extensions.get_extension_for_oid(
                 x509.OID_SUBJECT_KEY_IDENTIFIER
             )
+
+
+@pytest.mark.requires_backend_interface(interface=RSABackend)
+@pytest.mark.requires_backend_interface(interface=X509Backend)
+class TestKeyUsageExtension(object):
+    def test_no_key_usage(self, backend):
+        cert = _load_cert(
+            os.path.join("x509", "verisign_md2_root.pem"),
+            x509.load_pem_x509_certificate,
+            backend
+        )
+        ext = cert.extensions
+        with pytest.raises(x509.ExtensionNotFound) as exc:
+            ext.get_extension_for_oid(x509.OID_KEY_USAGE)
+
+        assert exc.value.oid == x509.OID_KEY_USAGE
+
+    def test_all_purposes(self, backend):
+        cert = _load_cert(
+            os.path.join(
+                "x509", "custom", "all_key_usages.pem"
+            ),
+            x509.load_pem_x509_certificate,
+            backend
+        )
+        extensions = cert.extensions
+        ext = extensions.get_extension_for_oid(x509.OID_KEY_USAGE)
+        assert ext is not None
+
+        ku = ext.value
+        assert ku.digital_signature is True
+        assert ku.content_commitment is True
+        assert ku.key_encipherment is True
+        assert ku.data_encipherment is True
+        assert ku.key_agreement is True
+        assert ku.key_cert_sign is True
+        assert ku.crl_sign is True
+        assert ku.encipher_only is True
+        assert ku.decipher_only is True
+
+    def test_key_cert_sign_crl_sign(self, backend):
+        cert = _load_cert(
+            os.path.join(
+                "x509", "PKITS_data", "certs", "pathLenConstraint6CACert.crt"
+            ),
+            x509.load_der_x509_certificate,
+            backend
+        )
+        ext = cert.extensions.get_extension_for_oid(x509.OID_KEY_USAGE)
+        assert ext is not None
+        assert ext.critical is True
+
+        ku = ext.value
+        assert ku.digital_signature is False
+        assert ku.content_commitment is False
+        assert ku.key_encipherment is False
+        assert ku.data_encipherment is False
+        assert ku.key_agreement is False
+        assert ku.key_cert_sign is True
+        assert ku.crl_sign is True