diff --git a/Lib/idlelib/FormatParagraph.py b/Lib/idlelib/FormatParagraph.py
index 2ac87e4..9b10c0a 100644
--- a/Lib/idlelib/FormatParagraph.py
+++ b/Lib/idlelib/FormatParagraph.py
@@ -188,7 +188,6 @@
     return m.group(1)
 
 if __name__ == "__main__":
-    from test import support; support.use_resources = ['gui']
     import unittest
     unittest.main('idlelib.idle_test.test_formatparagraph',
             verbosity=2, exit=False)
diff --git a/Lib/idlelib/IdleHistory.py b/Lib/idlelib/IdleHistory.py
index 931d5b2..078af29 100644
--- a/Lib/idlelib/IdleHistory.py
+++ b/Lib/idlelib/IdleHistory.py
@@ -100,7 +100,5 @@
         self.prefix = None
 
 if __name__ == "__main__":
-    from test import test_support as support
-    support.use_resources = ['gui']
     from unittest import main
     main('idlelib.idle_test.test_idlehistory', verbosity=2, exit=False)
diff --git a/Lib/idlelib/SearchEngine.py b/Lib/idlelib/SearchEngine.py
index 9517412..b3b7b4d 100644
--- a/Lib/idlelib/SearchEngine.py
+++ b/Lib/idlelib/SearchEngine.py
@@ -229,6 +229,5 @@
     return line, col
 
 if __name__ == "__main__":
-    from test import test_support; test_support.use_resources = ['gui']
     import unittest
     unittest.main('idlelib.idle_test.test_searchengine', verbosity=2, exit=False)
diff --git a/Lib/idlelib/idle_test/README.txt b/Lib/idlelib/idle_test/README.txt
index 6b92483..f6b6a21 100644
--- a/Lib/idlelib/idle_test/README.txt
+++ b/Lib/idlelib/idle_test/README.txt
@@ -26,7 +26,6 @@
 with xyz (lowercased) added after 'test_'.
 ---
 if __name__ == "__main__":
-    from test import support; support.use_resources = ['gui']
     import unittest
     unittest.main('idlelib.idle_test.test_', verbosity=2, exit=False)
 ---
@@ -34,12 +33,12 @@
 
 2. Gui Tests
 
-Gui tests need 'requires' and 'use_resources' from test.support
-(test.test_support in 2.7). A test is a gui test if it creates a Tk root or
-master object either directly or indirectly by instantiating a tkinter or
-idle class. For the benefit of buildbot machines that do not have a graphics
-screen, gui tests must be 'guarded' by "requires('gui')" in a setUp
-function or method. This will typically be setUpClass.
+Gui tests need 'requires' from test.support (test.test_support in 2.7). A
+test is a gui test if it creates a Tk root or master object either directly
+or indirectly by instantiating a tkinter or idle class. For the benefit of
+test processes that either have no graphical environment available or are not
+allowed to use it, gui tests must be 'guarded' by "requires('gui')" in a
+setUp function or method. This will typically be setUpClass.
 
 To avoid interfering with other gui tests, all gui objects must be destroyed
 and deleted by the end of the test.  If a widget, such as a Tk root, is created
@@ -57,11 +56,17 @@
         del cls.root
 ---
 
-Support.requires('gui') returns true if it is either called in a main module
-(which never happens on buildbots) or if use_resources contains 'gui'.
-Use_resources is set by test.regrtest but not by unittest. So when running
-tests in another module with unittest, we set it ourselves, as in the xyz.py
-template above.
+Support.requires('gui') causes the test(s) it guards to be skipped if any of
+a few conditions are met:
+ - The tests are being run by regrtest.py, and it was started without
+   enabling the "gui" resource with the "-u" command line option.
+ - The tests are being run on Windows by a service that is not allowed to
+   interact with the graphical environment.
+ - The tests are being run on Mac OSX in a process that cannot make a window
+   manager connection.
+ - tkinter.Tk cannot be successfully instantiated for some reason.
+ - test.support.use_resources has been set by something other than
+   regrtest.py and does not contain "gui".
 
 Since non-gui tests always run, but gui tests only sometimes, tests of non-gui
 operations should best avoid needing a gui. Methods that make incidental use of
@@ -88,8 +93,8 @@
 
 To run all idle_test/test_*.py tests, either interactively
 ('>>>', with unittest imported) or from a command line, use one of the
-following. (Notes: unittest does not run gui tests; in 2.7, 'test ' (with the
-space) is 'test.regrtest '; where present, -v and -ugui can be omitted.)
+following. (Notes: in 2.7, 'test ' (with the space) is 'test.regrtest ';
+where present, -v and -ugui can be omitted.)
 
 >>> unittest.main('idlelib.idle_test', verbosity=2, exit=False)
 python -m unittest -v idlelib.idle_test
@@ -98,13 +103,13 @@
 
 The idle tests are 'discovered' by idlelib.idle_test.__init__.load_tests,
 which is also imported into test.test_idle. Normally, neither file should be
-changed when working on individual test modules. The third command runs runs
+changed when working on individual test modules. The third command runs
 unittest indirectly through regrtest. The same happens when the entire test
 suite is run with 'python -m test'. So that command must work for buildbots
 to stay green. Idle tests must not disturb the environment in a way that
 makes other tests fail (issue 18081).
 
 To run an individual Testcase or test method, extend the dotted name given to
-unittest on the command line. (But gui tests will not this way.)
+unittest on the command line.
 
 python -m unittest -v idlelib.idle_test.test_xyz.Test_case.test_meth
diff --git a/Lib/lib-tk/test/runtktests.py b/Lib/lib-tk/test/runtktests.py
index aff6441..d4b1893 100644
--- a/Lib/lib-tk/test/runtktests.py
+++ b/Lib/lib-tk/test/runtktests.py
@@ -67,5 +67,4 @@
                 yield test
 
 if __name__ == "__main__":
-    test.test_support.use_resources = ['gui']
     test.test_support.run_unittest(*get_tests())
diff --git a/Lib/test/test_codecmaps_hk.py b/Lib/test/test_codecmaps_hk.py
index 7ba191b..3fd3eb8 100644
--- a/Lib/test/test_codecmaps_hk.py
+++ b/Lib/test/test_codecmaps_hk.py
@@ -16,5 +16,4 @@
     test_support.run_unittest(__name__)
 
 if __name__ == "__main__":
-    test_support.use_resources = ['urlfetch']
     test_main()
diff --git a/Lib/test/test_decimal.py b/Lib/test/test_decimal.py
index 87e8816..0902278 100644
--- a/Lib/test/test_decimal.py
+++ b/Lib/test/test_decimal.py
@@ -2271,7 +2271,7 @@
                                   "operation raises different flags depending on flags set: " +
                                   "expected %s, got %s" % (expected_flags, new_flags))
 
-def test_main(arith=False, verbose=None, todo_tests=None, debug=None):
+def test_main(arith=None, verbose=None, todo_tests=None, debug=None):
     """ Execute the tests.
 
     Runs all arithmetic tests if arith is True or if the "decimal" resource
@@ -2280,7 +2280,7 @@
 
     init()
     global TEST_ALL, DEBUG
-    TEST_ALL = arith or is_resource_enabled('decimal')
+    TEST_ALL = arith if arith is not None else is_resource_enabled('decimal')
     DEBUG = debug
 
     if todo_tests is None:
diff --git a/Lib/test/test_idle.py b/Lib/test/test_idle.py
index 66dd8af..9bd38fe 100644
--- a/Lib/test/test_idle.py
+++ b/Lib/test/test_idle.py
@@ -17,8 +17,4 @@
     support.run_unittest(unittest.TestLoader().loadTestsFromModule(idletest))
 
 if __name__ == '__main__':
-    # Until unittest supports resources, we emulate regrtest's -ugui
-    # so loaded tests run the same as if textually present here.
-    # If any Idle test ever needs another resource, add it to the list.
-    support.use_resources = ['gui']  # use_resources is initially None
     unittest.main(verbosity=2, exit=False)
diff --git a/Lib/test/test_imaplib.py b/Lib/test/test_imaplib.py
index 1beb39e..405b7ea 100644
--- a/Lib/test/test_imaplib.py
+++ b/Lib/test/test_imaplib.py
@@ -249,5 +249,4 @@
 
 
 if __name__ == "__main__":
-    support.use_resources = ['network']
     test_main()
diff --git a/Lib/test/test_support.py b/Lib/test/test_support.py
index b814952..e1ee3f5 100644
--- a/Lib/test/test_support.py
+++ b/Lib/test/test_support.py
@@ -346,21 +346,17 @@
     return _is_gui_available.result
 
 def is_resource_enabled(resource):
-    """Test whether a resource is enabled.  Known resources are set by
-    regrtest.py."""
-    return use_resources is not None and resource in use_resources
+    """Test whether a resource is enabled.
+
+    Known resources are set by regrtest.py.  If not running under regrtest.py,
+    all resources are assumed enabled unless use_resources has been set.
+    """
+    return use_resources is None or resource in use_resources
 
 def requires(resource, msg=None):
-    """Raise ResourceDenied if the specified resource is not available.
-
-    If the caller's module is __main__ then automatically return True.  The
-    possibility of False being returned occurs when regrtest.py is executing."""
+    """Raise ResourceDenied if the specified resource is not available."""
     if resource == 'gui' and not _is_gui_available():
         raise ResourceDenied(_is_gui_available.reason)
-    # see if the caller's module is __main__ - if so, treat as if
-    # the resource was set
-    if sys._getframe(1).f_globals.get("__name__") == "__main__":
-        return
     if not is_resource_enabled(resource):
         if msg is None:
             msg = "Use of the `%s' resource not enabled" % resource
diff --git a/Lib/test/test_tk.py b/Lib/test/test_tk.py
index 56eef47..f3e264b 100644
--- a/Lib/test/test_tk.py
+++ b/Lib/test/test_tk.py
@@ -12,16 +12,10 @@
 with test_support.DirsOnSysPath(lib_tk_test):
     import runtktests
 
-def test_main(enable_gui=False):
-    if enable_gui:
-        if test_support.use_resources is None:
-            test_support.use_resources = ['gui']
-        elif 'gui' not in test_support.use_resources:
-            test_support.use_resources.append('gui')
-
+def test_main():
     with test_support.DirsOnSysPath(lib_tk_test):
         test_support.run_unittest(
             *runtktests.get_tests(text=False, packages=['test_tkinter']))
 
 if __name__ == '__main__':
-    test_main(enable_gui=True)
+    test_main()
diff --git a/Lib/test/test_ttk_guionly.py b/Lib/test/test_ttk_guionly.py
index 47ddefb..caa6930 100644
--- a/Lib/test/test_ttk_guionly.py
+++ b/Lib/test/test_ttk_guionly.py
@@ -22,13 +22,7 @@
     # assuming ttk is not available
     raise unittest.SkipTest("ttk not available: %s" % msg)
 
-def test_main(enable_gui=False):
-    if enable_gui:
-        if test_support.use_resources is None:
-            test_support.use_resources = ['gui']
-        elif 'gui' not in test_support.use_resources:
-            test_support.use_resources.append('gui')
-
+def test_main():
     with test_support.DirsOnSysPath(lib_tk_test):
         from test_ttk.support import get_tk_root
         try:
@@ -38,4 +32,4 @@
             get_tk_root().destroy()
 
 if __name__ == '__main__':
-    test_main(enable_gui=True)
+    test_main()
diff --git a/Misc/NEWS b/Misc/NEWS
index 8320b0e..4cf70a6 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -46,6 +46,9 @@
 Tests
 -----
 
+- Issue #18492: All resources are now allowed when tests are not run by
+  regrtest.py.
+
 - Issue #21605: Added tests for Tkinter images.
 
 - Issue #21493: Added test for ntpath.expanduser().  Original patch by