diff --git a/.travis.yml b/.travis.yml
index c4d0bfb2..a0a0c8bb 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -9,7 +9,10 @@ python:
   - "3.7-dev"
   - "nightly"
 # command to install dependencies
-before_install: "sudo apt-get update; sudo apt-get install -y python3-nose python3-flask python3-stem python3-pyqt5"
-install: ""
+# before_install: "sudo apt-get update; sudo apt-get install -y python3-nose python3-flask python3-stem python3-pyqt5"
+install:
+  - pip install Flask==0.12 stem==1.5.4 pytest-cov coveralls
 # command to run tests
-script: nosetests3
+script: pytest test/ --cov test/
+after_success:
+  - coveralls
diff --git a/test/__init__.py b/test/__init__.py
new file mode 100644
index 00000000..e69de29b
diff --git a/test/onionshare_common_test.py b/test/onionshare_common_test.py
index 3bc4b9d6..8b5923af 100644
--- a/test/onionshare_common_test.py
+++ b/test/onionshare_common_test.py
@@ -16,20 +16,440 @@ GNU General Public License for more details.
 You should have received a copy of the GNU General Public License
 along with this program.  If not, see <http://www.gnu.org/licenses/>.
 """
+
+import contextlib
+import inspect
+import io
+import os
+import platform
+import random
+import re
+import shutil
 import socket
+import sys
+import tempfile
+import time
+import zipfile
+
+import pytest
+
 from onionshare import common
 
+DEFAULT_ZW_FILENAME_REGEX = re.compile(r'^onionshare_[a-z2-7]{6}.zip$')
+# TODO: use re.VERBOSE on LOG_MSG_REGEX for readability?
+LOG_MSG_REGEX = re.compile(r'^\[Jun 06 2013 11:05:00\] TestModule\.<function test_log\.<locals>\.test_func at 0x[a-f0-9]+>(: TEST_MSG)?$')
+RANDOM_STR_REGEX = re.compile(r'^[a-z2-7]+$')
+SLUG_REGEX = re.compile(r'^([a-z]+)(-[a-z]+)?-([a-z]+)(-[a-z]+)?$')
 
-def test_get_platform_returns_platform_system():
-    """get_platform() returns platform.system() when ONIONSHARE_PLATFORM is not defined"""
-    p = common.platform.system
-    common.platform.system = lambda: 'Sega Saturn'
-    assert common.get_platform() == 'Sega Saturn'
-    common.platform.system = p
 
-def test_get_available_port_returns_an_open_port():
-    """get_available_port() should return an open port within the range"""
-    for i in range(100):
-        port = common.get_available_port(1024, 2048)
-        assert 1024 <= port <= 2048
-        socket.socket().bind(("127.0.0.1", port))
+# #################################################
+# FIXTURES
+# #################################################
+
+# TODO: separate fixtures into a separate file?
+# TODO: comment fixtures properly
+
+
+@pytest.fixture(scope='session')
+def custom_zw():
+    # TODO: revert to old style setup/teardown instead of yield
+    zw = common.ZipWriter(zip_filename=common.random_string(4, 6))
+    yield zw
+    zw.close()
+    os.remove(zw.zip_filename)
+
+
+@pytest.fixture(scope='session')
+def default_zw():
+    # TODO: revert to old style setup/teardown instead of yield
+    zw = common.ZipWriter()
+    yield zw
+    zw.close()
+    os.remove(zw.zip_filename)
+
+
+@pytest.fixture
+def platform_darwin(monkeypatch):
+    monkeypatch.setattr(platform, 'system', lambda: 'Darwin')
+
+
+@pytest.fixture
+def platform_linux(monkeypatch):
+    monkeypatch.setattr(platform, 'system', lambda: 'Linux')
+
+
+@pytest.fixture
+def platform_windows(monkeypatch):
+    monkeypatch.setattr(platform, 'system', lambda: 'Windows')
+
+
+@pytest.fixture
+def sys_argv_sys_prefix(monkeypatch):
+    monkeypatch.setattr(sys, 'argv', [sys.prefix])
+
+
+@pytest.fixture
+def sys_frozen(monkeypatch):
+    monkeypatch.setattr(sys, 'frozen', True, raising=False)
+
+
+@pytest.fixture
+def sys_meipass(monkeypatch):
+    monkeypatch.setattr(
+        sys, '_MEIPASS', os.path.expanduser('~'), raising=False)
+
+
+@pytest.fixture
+def sys_onionshare_dev_mode(monkeypatch):
+    monkeypatch.setattr(sys, 'onionshare_dev_mode', True, raising=False)
+
+
+@pytest.fixture
+def time_100(monkeypatch):
+    monkeypatch.setattr(time, 'time', lambda: 100)
+
+
+@pytest.fixture
+def time_strftime(monkeypatch):
+    monkeypatch.setattr(time, 'strftime', lambda _: 'Jun 06 2013 11:05:00')
+
+
+# #################################################
+# TESTS
+# #################################################
+
+
+class TestBuildSlug(object):
+    @pytest.mark.parametrize('test_input,expected', [
+        # VALID, two lowercase words, separated by a hyphen
+        ('syrup-enzyme', True),
+        ('caution-friday', True),
+
+        # VALID, two lowercase words, with one hyphenated compound word
+        ('drop-down-thimble', True),
+        ('unmixed-yo-yo', True),
+
+        # VALID, two lowercase hyphenated compound words, separated by hyphen
+        ('yo-yo-drop-down', True),
+        ('felt-tip-t-shirt', True),
+        ('hello-world', True),
+
+        # INVALID
+        ('Upper-Case', False),
+        ('digits-123', False),
+        ('too-many-hyphens-', False),
+        ('symbols-!@#$%', False)
+    ])
+    def test_build_slug_regex(self, test_input, expected):
+        """ Test that `SLUG_REGEX` accounts for the following patterns
+
+        There are a few hyphenated words in `wordlist.txt`:
+            * drop-down
+            * felt-tip
+            * t-shirt
+            * yo-yo
+
+        These words cause a few extra potential slug patterns:
+            * word-word
+            * hyphenated-word-word
+            * word-hyphenated-word
+            * hyphenated-word-hyphenated-word
+        """
+
+        assert bool(SLUG_REGEX.match(test_input)) == expected
+
+    def test_build_slug_unique(self):
+        assert common.build_slug() != common.build_slug()
+
+
+@pytest.mark.parametrize('directory_size', (5, 500, 5000))
+def test_dir_size(directory_size):
+    """ dir_size() should return the total size (in bytes) of all files
+    in a particular directory.
+
+    This test creates a temporary directory with a single file of a
+    particular size. After the test is complete, it deletes the
+    temporary directory.
+    """
+
+    # TODO: use helper function to create temporary file?
+    tmp_dir = tempfile.mkdtemp()
+    with tempfile.NamedTemporaryFile(dir=tmp_dir, delete=False) as tmp_file:
+        tmp_file.write(b'*' * directory_size)
+
+    # tempfile.TemporaryDirectory raised error when given to `dir_size`
+    assert common.dir_size(tmp_dir) == directory_size
+    shutil.rmtree(tmp_dir)
+
+
+class TestEstimatedTimeRemaining(object):
+    @pytest.mark.parametrize('test_input,expected', (
+        ((2, 676, 12), '8h14m16s'),
+        ((14, 1049, 30), '1h26m15s'),
+        ((21, 450, 1), '33m42s'),
+        ((31, 1115, 80), '11m39s'),
+        ((336, 989, 32), '2m12s'),
+        ((603, 949, 38), '36s'),
+        ((971, 1009, 83), '1s')
+    ))
+    def test_estimated_time_remaining(self, test_input, expected, time_100):
+        assert common.estimated_time_remaining(*test_input) == expected
+
+    def test_estimated_time_remaining_time_elapsed_zero(self, time_100):
+        """ estimated_time_remaining() raises a ZeroDivisionError if
+        `time_elapsed` == 0
+        """
+
+        with pytest.raises(ZeroDivisionError):
+            common.estimated_time_remaining(10, 20, 100)
+
+    def test_estimated_time_remaining_download_rate_zero(self, time_100):
+        """ estimated_time_remaining() raises a ZeroDivision error if
+        `download_rate` == 0
+        """
+
+        with pytest.raises(ZeroDivisionError):
+            common.estimated_time_remaining(0, 37, 99)
+
+
+@pytest.mark.parametrize('test_input,expected', [
+    (0, '0s'),
+    (26, '26s'),
+    (60, '1m'),
+    (947.35, '15m47s'),
+    (1847, '30m47s'),
+    (2193.94, '36m34s'),
+    (3600, '1h'),
+    (13426.83, '3h43m47s'),
+    (16293, '4h31m33s'),
+    (18392.14, '5h6m32s'),
+    (86400, '1d'),
+    (129674, '1d12h1m14s'),
+    (56404.12, '15h40m4s'),
+])
+def test_format_seconds(test_input, expected):
+    assert common.format_seconds(test_input) == expected
+
+
+@pytest.mark.parametrize('port_min,port_max', (
+    (random.randint(1024, 1500),
+     random.randint(1800, 2048)) for _ in range(100)
+))
+def test_get_available_port_returns_an_open_port(port_min, port_max):
+    """ get_available_port() should return an open port within the range """
+
+    port = common.get_available_port(port_min, port_max)
+    assert port_min <= port <= port_max
+    with socket.socket() as tmpsock:
+        tmpsock.bind(('127.0.0.1', port))
+
+
+# TODO: is there a way to parametrize (fixture, expected)?
+class TestGetPlatform(object):
+    def test_darwin(self, platform_darwin):
+        assert common.get_platform() == 'Darwin'
+
+    def test_linux(self, platform_linux):
+        assert common.get_platform() == 'Linux'
+
+    def test_windows(self, platform_windows):
+        assert common.get_platform() == 'Windows'
+
+
+# TODO: double-check these tests
+class TestGetResourcePath(object):
+    def test_onionshare_dev_mode(self, sys_onionshare_dev_mode):
+        prefix = os.path.join(
+            os.path.dirname(
+                os.path.dirname(
+                    os.path.abspath(
+                        inspect.getfile(
+                            inspect.currentframe())))), 'share')
+        assert (
+            common.get_resource_path(os.path.join(prefix, 'test_filename')) ==
+            os.path.join(prefix, 'test_filename'))
+
+    def test_linux(self, platform_linux, sys_argv_sys_prefix):
+        prefix = os.path.join(sys.prefix, 'share/onionshare')
+        assert (
+            common.get_resource_path(os.path.join(prefix, 'test_filename')) ==
+            os.path.join(prefix, 'test_filename'))
+
+    def test_frozen_darwin(self, platform_darwin, sys_frozen, sys_meipass):
+        prefix = os.path.join(sys._MEIPASS, 'share')
+        assert (
+            common.get_resource_path(os.path.join(prefix, 'test_filename')) ==
+            os.path.join(prefix, 'test_filename'))
+
+    def test_frozen_windows(self, platform_windows, sys_frozen):
+        prefix = os.path.join(os.path.dirname(sys.executable), 'share')
+        assert (
+            common.get_resource_path(os.path.join(prefix, 'test_filename')) ==
+            os.path.join(prefix, 'test_filename'))
+
+
+class TestGetTorPaths(object):
+    # @pytest.mark.skipif(sys.platform != 'Darwin', reason='requires MacOS') ?
+    def test_get_tor_paths_darwin(self, platform_darwin):
+        base_path = os.path.dirname(os.path.dirname(
+            os.path.dirname(common.get_resource_path(''))))
+        tor_path = os.path.join(base_path, 'Resources', 'Tor', 'tor')
+        tor_geo_ip_file_path = os.path.join(
+            base_path, 'Resources', 'Tor', 'geoip')
+        tor_geo_ipv6_file_path = os.path.join(
+            base_path, 'Resources', 'Tor', 'geoip6')
+        assert (common.get_tor_paths() ==
+                (tor_path, tor_geo_ip_file_path, tor_geo_ipv6_file_path))
+
+    # @pytest.mark.skipif(sys.platform != 'Linux', reason='requires Linux') ?
+    def test_get_tor_paths_linux(self, platform_linux):
+        assert (common.get_tor_paths() ==
+                ('/usr/bin/tor', '/usr/share/tor/geoip', '/usr/share/tor/geoip6'))
+
+    # @pytest.mark.skipif(sys.platform != 'Windows', reason='requires Windows') ?
+    def test_get_tor_paths_windows(self, platform_windows):
+        base_path = os.path.join(
+            os.path.dirname(os.path.dirname(common.get_resource_path(''))), 'tor')
+        tor_path = os.path.join(os.path.join(base_path, 'Tor'), "tor.exe")
+        tor_geo_ip_file_path = os.path.join(
+            os.path.join(os.path.join(base_path, 'Data'), 'Tor'), 'geoip')
+        tor_geo_ipv6_file_path = os.path.join(
+            os.path.join(os.path.join(base_path, 'Data'), 'Tor'), 'geoip6')
+        assert (common.get_tor_paths() ==
+                (tor_path, tor_geo_ip_file_path, tor_geo_ipv6_file_path))
+
+
+def test_get_version():
+    with open(common.get_resource_path('version.txt')) as f:
+        version = f.read().strip()
+
+    assert version == common.get_version()
+
+
+@pytest.mark.parametrize('test_input,expected', (
+    (1024 ** 0, '1.0 B'),
+    (1024 ** 1, '1.0 KiB'),
+    (1024 ** 2, '1.0 MiB'),
+    (1024 ** 3, '1.0 GiB'),
+    (1024 ** 4, '1.0 TiB'),
+    (1024 ** 5, '1.0 PiB'),
+    (1024 ** 6, '1.0 EiB'),
+    (1024 ** 7, '1.0 ZiB'),
+    (1024 ** 8, '1.0 YiB')
+))
+def test_human_readable_filesize(test_input, expected):
+    assert common.human_readable_filesize(test_input) == expected
+
+
+def test_log(time_strftime):
+    def test_func():
+        pass
+
+    # TODO: create and use `set_debug` fixture instead?
+    common.set_debug(True)
+
+    # From: https://stackoverflow.com/questions/1218933
+    with io.StringIO() as buf, contextlib.redirect_stdout(buf):
+        common.log('TestModule', test_func)
+        common.log('TestModule', test_func, 'TEST_MSG')
+        output = buf.getvalue()
+
+    common.set_debug(False)
+
+    line_one, line_two, _ = output.split('\n')
+    assert LOG_MSG_REGEX.match(line_one)
+    assert LOG_MSG_REGEX.match(line_two)
+
+
+@pytest.mark.parametrize('test_input,expected', (
+    (common.random_string(
+        random.randint(2, 50),
+        random.choice((None, random.randint(2, 50)))),
+     True) for _ in range(50)
+))
+def test_random_string_regex(test_input, expected):
+    assert bool(RANDOM_STR_REGEX.match(test_input)) == expected
+
+
+# TODO: create and use `set_debug` fixture instead?
+class TestSetDebug(object):
+    def test_set_debug_true(self):
+        common.set_debug(True)
+        assert common.debug is True
+
+    def test_set_debug_false(self):
+        common.set_debug(False)
+        assert common.debug is False
+
+
+# TODO: ZipWriter doesn't enforce the `.zip` extension with custom filename
+class TestDefaultZipWriter(object):
+    def test_zw_filename(self, default_zw):
+        zw_filename = os.path.basename(default_zw.zip_filename)
+        assert bool(DEFAULT_ZW_FILENAME_REGEX.match(zw_filename))
+
+    def test_zipfile_filename_matches_zipwriter_filename(self, default_zw):
+        assert default_zw.z.filename == default_zw.zip_filename
+
+    def test_zipfile_allow_zip64(self, default_zw):
+        assert default_zw.z._allowZip64 is True
+
+    def test_zipfile_mode(self, default_zw):
+        assert default_zw.z.mode == 'w'
+
+    def test_callback(self, default_zw):
+        assert default_zw.processed_size_callback(None) is None
+
+    def test_add_file(self, default_zw):
+        tmp_file_size = 1000
+        # TODO: use helper function to create temporary file?
+        with tempfile.NamedTemporaryFile(delete=False) as tmp_file:
+            tmp_file.write(b'*' * tmp_file_size)
+
+        tmp_file_path = tmp_file.name
+        default_zw.add_file(tmp_file_path)
+
+        zipfile_info = default_zw.z.getinfo(os.path.basename(tmp_file_path))
+        assert zipfile_info.compress_type == zipfile.ZIP_DEFLATED
+        assert zipfile_info.file_size == tmp_file_size
+        assert zipfile_info.is_dir() is False
+
+        os.remove(tmp_file_path)
+        assert os.path.exists(tmp_file_path) is False
+
+    def test_add_directory(self, default_zw):
+        directory_size = 1000
+        tmp_dir = create_temporary_directory(directory_size)
+        current_size = default_zw._size
+        default_zw.add_dir(tmp_dir)
+
+        assert default_zw._size == current_size + directory_size
+        shutil.rmtree(tmp_dir)
+        assert os.path.exists(tmp_dir) is False
+
+
+def test_zip_writer_custom_filename(custom_zw):
+    assert bool(RANDOM_STR_REGEX.match(custom_zw.zip_filename))
+
+
+def create_temporary_directory(directory_size):
+    """ Create a temporary directory with a single file of a
+    particular size. Return directory path as a string
+    """
+
+    tmp_dir = tempfile.mkdtemp()
+    # create_temporary_file(directory=tmp_dir)
+
+    with tempfile.NamedTemporaryFile(dir=tmp_dir, delete=False) as tmp_file:
+        tmp_file.write(b'*' * directory_size)
+
+    return tmp_dir
+
+
+# TODO: rewrite this helper function to DRY up tests that use temporary files
+# def create_temporary_file(directory=None, delete=False, file_size=100):
+#     if file_size <= 0:
+#         file_size = 100
+#     with tempfile.NamedTemporaryFile(dir=directory, delete=delete) as tmp_file:
+#         tmp_file.write(b'*' * file_size)
+#     return tmp_file.name