diff --git a/doc/api/next_api_changes/2018-05-06-DS.rst b/doc/api/next_api_changes/2018-05-06-DS.rst
new file mode 100644
index 000000000000..d96128486ff8
--- /dev/null
+++ b/doc/api/next_api_changes/2018-05-06-DS.rst
@@ -0,0 +1,7 @@
+Default value for ``block`` keyword arg standardised to ``True``
+----------------------------------------------------------------
+
+The ``block`` keyword argument that can be supplied to `.pyplot.show()` now
+takes a default value of ``True`` instead of ``None``. The behaviour is
+otherwise the same, and the `.backend_bases.FigureManagerBase.show()` has
+also had this change applied.
diff --git a/lib/matplotlib/backend_bases.py b/lib/matplotlib/backend_bases.py
index 030e21f7687f..8d076f2755d6 100644
--- a/lib/matplotlib/backend_bases.py
+++ b/lib/matplotlib/backend_bases.py
@@ -177,12 +177,11 @@ def draw_if_interactive(cls):
                 cls.trigger_manager_draw(manager)
 
     @classmethod
-    def show(cls, block=None):
+    def show(cls, block=True):
         """Show all figures.
 
-        `show` blocks by calling `mainloop` if *block* is ``True``, or if it
-        is ``None`` and we are neither in IPython's ``%pylab`` mode, nor in
-        `interactive` mode.
+        `show` blocks by calling `mainloop` if *block* is ``True`` and we are
+        neither in IPython's ``%pylab`` mode, nor in `interactive` mode.
         """
         managers = Gcf.get_all_fig_managers()
         if not managers:
@@ -198,7 +197,7 @@ def show(cls, block=None):
                 return
         if cls.mainloop is None:
             return
-        if block is None:
+        if block:
             # Hack: Are we in IPython's pylab mode?
             from matplotlib import pyplot
             try:
@@ -210,8 +209,6 @@ def show(cls, block=None):
             block = not ipython_pylab and not is_interactive()
             # TODO: The above is a hack to get the WebAgg backend working with
             # ipython's `%pylab` mode until proper integration is implemented.
-            if get_backend() == "WebAgg":
-                block = True
         if block:
             cls.mainloop()
 
@@ -244,7 +241,7 @@ class ShowBase(_Backend):
     Subclass must override mainloop() method.
     """
 
-    def __call__(self, block=None):
+    def __call__(self, block=True):
         return self.show(block=block)
 
 
@@ -2635,7 +2632,7 @@ def notify_axes_change(fig):
             if self.toolmanager is None and self.toolbar is not None:
                 self.toolbar.update()
 
-    def show(self):
+    def show(self, block=True):
         """
         For GUI backends, show the figure window and redraw.
         For non-GUI backends, raise an exception to be caught
diff --git a/lib/matplotlib/backends/backend_nbagg.py b/lib/matplotlib/backends/backend_nbagg.py
index 16d3d51e0a74..7300317675d7 100644
--- a/lib/matplotlib/backends/backend_nbagg.py
+++ b/lib/matplotlib/backends/backend_nbagg.py
@@ -242,7 +242,7 @@ def trigger_manager_draw(manager):
         manager.show()
 
     @staticmethod
-    def show(*args, **kwargs):
+    def show(block=True):
         ## TODO: something to do when keyword block==False ?
         from matplotlib._pylab_helpers import Gcf
 
diff --git a/lib/matplotlib/backends/backend_template.py b/lib/matplotlib/backends/backend_template.py
index 384a69850230..b7e44084fc46 100644
--- a/lib/matplotlib/backends/backend_template.py
+++ b/lib/matplotlib/backends/backend_template.py
@@ -172,7 +172,7 @@ def draw_if_interactive():
     """
 
 
-def show(block=None):
+def show(block=True):
     """
     For image backends - is not required
     For GUI backends - show() is usually the last line of a pylab script and
diff --git a/lib/matplotlib/backends/backend_webagg.py b/lib/matplotlib/backends/backend_webagg.py
index 18a995b97619..2b9bae78a030 100644
--- a/lib/matplotlib/backends/backend_webagg.py
+++ b/lib/matplotlib/backends/backend_webagg.py
@@ -319,7 +319,10 @@ def trigger_manager_draw(manager):
         manager.canvas.draw_idle()
 
     @staticmethod
-    def show():
+    def show(block=True):
+        if not block:
+            warnings.warn('The webagg backend does not support "block=False"')
+            block = True
         WebAggApplication.initialize()
 
         url = "http://127.0.0.1:{port}{prefix}".format(
diff --git a/lib/matplotlib/pyplot.py b/lib/matplotlib/pyplot.py
index 2725094fe077..e4fd8ecbd9ed 100644
--- a/lib/matplotlib/pyplot.py
+++ b/lib/matplotlib/pyplot.py
@@ -231,9 +231,10 @@ def switch_backend(newbackend):
     _backend_mod, new_figure_manager, draw_if_interactive, _show = pylab_setup()
 
 
-def show(*args, **kw):
+def show(block=True):
     """
     Display a figure.
+
     When running in ipython with its pylab mode, display all
     figures and return to the ipython prompt.
 
@@ -243,12 +244,14 @@ def show(*args, **kw):
     non-interactive to interactive mode (not recommended).  In
     that case it displays the figures but does not block.
 
-    A single experimental keyword argument, *block*, may be
-    set to True or False to override the blocking behavior
-    described above.
+    Parameters
+    ----------
+    block : bool, optional
+        Experimental; set to ``False`` to override the blocking
+        behavior described above.
     """
     global _show
-    return _show(*args, **kw)
+    return _show(block=block)
 
 
 def isinteractive():