diff --git a/README.rst b/README.rst
index 77af67e..32887c5 100644
--- a/README.rst
+++ b/README.rst
@@ -293,8 +293,11 @@ Vladiate comes with the following input types:
 
 *class* ``S3File``
 
-  Read from a file in S3. Uses the `boto <https://github.com/boto/boto>`_
-  library. Optionally can specify either a full path, or a bucket/key pair.
+  Read from a file in S3. Optionally can specify either a full path, or a
+  bucket/key pair.
+
+  Requires the `boto <https://github.com/boto/boto>`_ library, which should be
+  installed via ``pip install vladiate[s3]``.
 
   :``path=None``:
       A full S3 filepath (e.g., ``s3://foo.bar/path/to/file.csv``)
diff --git a/setup.py b/setup.py
index b5c8c72..67448c1 100644
--- a/setup.py
+++ b/setup.py
@@ -4,7 +4,7 @@
 from setuptools import setup, find_packages
 from setuptools.command.test import test as TestCommand
 
-__version__ = '0.0.18'
+__version__ = '0.0.19'
 
 
 class PyTest(TestCommand):
@@ -60,7 +60,8 @@ def readme():
     packages=find_packages(exclude=['examples', 'tests']),
     include_package_data=True,
     zip_safe=False,
-    install_requires=['boto'],
+    install_requires=[],
+    extras_require={'s3': ['boto']},
     tests_require=['pretend', 'pytest', 'flake8'],
     cmdclass={'test': PyTest},
     entry_points={
diff --git a/tox.ini b/tox.ini
index 80f6311..fa4df7a 100644
--- a/tox.ini
+++ b/tox.ini
@@ -19,7 +19,7 @@ basepython = python3.6
 commands = flake8
 
 [testenv]
-commands = coverage run --source=vladiate setup.py test
+commands = coverage run --append --source=vladiate setup.py test
 deps =
     pytest
     coverage
diff --git a/vladiate/exceptions.py b/vladiate/exceptions.py
index 46b7986..500977e 100644
--- a/vladiate/exceptions.py
+++ b/vladiate/exceptions.py
@@ -8,3 +8,12 @@ class BadValidatorException(Exception):
     def __init__(self, extra):
         message = 'Row does not contain the following unique_with fields: {}'
         super(BadValidatorException, self).__init__(message.format(extra))
+
+
+class MissingExtraException(Exception):
+    ''' Thrown when an extra dependency is missing '''
+    def __init__(self):
+        super(MissingExtraException, self).__init__(
+            'The `s3` extra is required to use the `S3File` class. Install'
+            ' it via `pip install vladiate[s3]`.'
+        )
diff --git a/vladiate/inputs.py b/vladiate/inputs.py
index 4639604..c86b26c 100644
--- a/vladiate/inputs.py
+++ b/vladiate/inputs.py
@@ -1,5 +1,4 @@
 import io
-import boto
 try:
     from urlparse import urlparse
 except:
@@ -9,6 +8,8 @@
 except:
     from io import StringIO
 
+from vladiate.exceptions import MissingExtraException
+
 
 class VladInput(object):
     ''' A generic input class '''
@@ -41,6 +42,15 @@ class S3File(VladInput):
     ''' Read from a file in S3 '''
 
     def __init__(self, path=None, bucket=None, key=None):
+        try:
+            import boto  # noqa
+            self.boto = boto
+        except:
+            # 2.7 workaround, should just be `raise Exception() from None`
+            exc = MissingExtraException()
+            exc.__context__ = None
+            raise exc
+
         if path and not any((bucket, key)):
             self.path = path
             parse_result = urlparse(path)
@@ -57,7 +67,7 @@ def __init__(self, path=None, bucket=None, key=None):
             )
 
     def open(self):
-        s3 = boto.connect_s3()
+        s3 = self.boto.connect_s3()
         bucket = s3.get_bucket(self.bucket)
         key = bucket.new_key(self.key)
         contents = key.get_contents_as_string()
diff --git a/vladiate/test/test_inputs.py b/vladiate/test/test_inputs.py
index b35843c..b2aa700 100644
--- a/vladiate/test/test_inputs.py
+++ b/vladiate/test/test_inputs.py
@@ -1,15 +1,32 @@
 import pytest
 from pretend import stub, call, call_recorder
 
+from ..exceptions import MissingExtraException
 from ..inputs import S3File, StringIO, String, VladInput
 from ..vlad import Vlad
 
 
+def mock_boto(result):
+    try:
+        import builtins
+    except:
+        import __builtin__ as builtins
+    realimport = builtins.__import__
+
+    def badimport(name, *args, **kwargs):
+        if name == 'boto':
+            return result()
+        return realimport(name, *args, **kwargs)
+
+    builtins.__import__ = badimport
+
+
 @pytest.mark.parametrize('kwargs', [
     ({'path': 's3://some.bucket/some/s3/key.csv'}),
     ({'bucket': 'some.bucket', 'key': '/some/s3/key.csv'}),
 ])
 def test_s3_input_works(kwargs):
+    mock_boto(lambda: stub())
     S3File(**kwargs)
 
 
@@ -21,6 +38,7 @@ def test_s3_input_works(kwargs):
     ({'key': '/some/s3/key.csv'}),
 ])
 def test_s3_input_fails(kwargs):
+    mock_boto(lambda: stub())
     with pytest.raises(ValueError):
         S3File(**kwargs)
 
@@ -35,7 +53,7 @@ def test_string_input_works(kwargs):
     assert Vlad(source=source, validators=validators).validate()
 
 
-def test_open_s3file(monkeypatch):
+def test_open_s3file():
     new_key = call_recorder(lambda *args, **kwargs: stub(
         get_contents_as_string=lambda: 'contents'.encode()
     ))
@@ -44,9 +62,10 @@ def test_open_s3file(monkeypatch):
 
     mock_boto = stub(connect_s3=lambda: stub(get_bucket=get_bucket))
 
-    monkeypatch.setattr('vladiate.inputs.boto', mock_boto)
+    s3file = S3File('s3://some.bucket/some/s3/key.csv')
+    s3file.boto = mock_boto
 
-    result = S3File('s3://some.bucket/some/s3/key.csv').open()
+    result = s3file.open()
 
     assert get_bucket.calls == [
         call('some.bucket')
@@ -77,3 +96,13 @@ def __init__(self):
 
     with pytest.raises(NotImplementedError):
         repr(PartiallyImplemented())
+
+
+def test_s3file_raises_when_no_boto():
+    def import_result():
+        raise ImportError
+
+    mock_boto(import_result)
+
+    with pytest.raises(MissingExtraException):
+        S3File()