diff --git a/Doc/library/xml.etree.elementtree.rst b/Doc/library/xml.etree.elementtree.rst
index 4a7cf6f..9bee0ea 100644
--- a/Doc/library/xml.etree.elementtree.rst
+++ b/Doc/library/xml.etree.elementtree.rst
@@ -594,6 +594,7 @@
 
 
 .. function:: tostring(element, encoding="us-ascii", method="xml", *, \
+                       xml_declaration=None, default_namespace=None,
                        short_empty_elements=True)
 
    Generates a string representation of an XML element, including all
@@ -601,14 +602,19 @@
    the output encoding (default is US-ASCII).  Use ``encoding="unicode"`` to
    generate a Unicode string (otherwise, a bytestring is generated).  *method*
    is either ``"xml"``, ``"html"`` or ``"text"`` (default is ``"xml"``).
-   *short_empty_elements* has the same meaning as in :meth:`ElementTree.write`.
-   Returns an (optionally) encoded string containing the XML data.
+   *xml_declaration*, *default_namespace* and *short_empty_elements* has the same
+   meaning as in :meth:`ElementTree.write`. Returns an (optionally) encoded string
+   containing the XML data.
 
    .. versionadded:: 3.4
       The *short_empty_elements* parameter.
 
+   .. versionadded:: 3.8
+      The *xml_declaration* and *default_namespace* parameters.
+
 
 .. function:: tostringlist(element, encoding="us-ascii", method="xml", *, \
+                           xml_declaration=None, default_namespace=None,
                            short_empty_elements=True)
 
    Generates a string representation of an XML element, including all
@@ -616,16 +622,19 @@
    the output encoding (default is US-ASCII).  Use ``encoding="unicode"`` to
    generate a Unicode string (otherwise, a bytestring is generated).  *method*
    is either ``"xml"``, ``"html"`` or ``"text"`` (default is ``"xml"``).
-   *short_empty_elements* has the same meaning as in :meth:`ElementTree.write`.
-   Returns a list of (optionally) encoded strings containing the XML data.
-   It does not guarantee any specific sequence, except that
-   ``b"".join(tostringlist(element)) == tostring(element)``.
+   *xml_declaration*, *default_namespace* and *short_empty_elements* has the same
+   meaning as in :meth:`ElementTree.write`. Returns a list of (optionally) encoded
+   strings containing the XML data. It does not guarantee any specific sequence,
+   except that ``b"".join(tostringlist(element)) == tostring(element)``.
 
    .. versionadded:: 3.2
 
    .. versionadded:: 3.4
       The *short_empty_elements* parameter.
 
+   .. versionadded:: 3.8
+      The *xml_declaration* and *default_namespace* parameters.
+
 
 .. function:: XML(text, parser=None)
 
diff --git a/Lib/test/test_xml_etree.py b/Lib/test/test_xml_etree.py
index 8a7ec00..bdcd4e0 100644
--- a/Lib/test/test_xml_etree.py
+++ b/Lib/test/test_xml_etree.py
@@ -9,6 +9,7 @@
 import functools
 import html
 import io
+import locale
 import operator
 import pickle
 import sys
@@ -756,6 +757,128 @@
         elem = ET.fromstring("<html><body>text</body></html>")
         self.assertEqual(ET.tostring(elem), b'<html><body>text</body></html>')
 
+    def test_tostring_default_namespace(self):
+        elem = ET.XML('<body xmlns="http://effbot.org/ns"><tag/></body>')
+        self.assertEqual(
+            ET.tostring(elem, encoding='unicode'),
+            '<ns0:body xmlns:ns0="http://effbot.org/ns"><ns0:tag /></ns0:body>'
+        )
+        self.assertEqual(
+            ET.tostring(elem, encoding='unicode', default_namespace='http://effbot.org/ns'),
+            '<body xmlns="http://effbot.org/ns"><tag /></body>'
+        )
+
+    def test_tostring_default_namespace_different_namespace(self):
+        elem = ET.XML('<body xmlns="http://effbot.org/ns"><tag/></body>')
+        self.assertEqual(
+            ET.tostring(elem, encoding='unicode', default_namespace='foobar'),
+            '<ns1:body xmlns="foobar" xmlns:ns1="http://effbot.org/ns"><ns1:tag /></ns1:body>'
+        )
+
+    def test_tostring_default_namespace_original_no_namespace(self):
+        elem = ET.XML('<body><tag/></body>')
+        EXPECTED_MSG = '^cannot use non-qualified names with default_namespace option$'
+        with self.assertRaisesRegex(ValueError, EXPECTED_MSG):
+            ET.tostring(elem, encoding='unicode', default_namespace='foobar')
+
+    def test_tostring_no_xml_declaration(self):
+        elem = ET.XML('<body><tag/></body>')
+        self.assertEqual(
+            ET.tostring(elem, encoding='unicode'),
+            '<body><tag /></body>'
+        )
+
+    def test_tostring_xml_declaration(self):
+        elem = ET.XML('<body><tag/></body>')
+        self.assertEqual(
+            ET.tostring(elem, encoding='utf8', xml_declaration=True),
+            b"<?xml version='1.0' encoding='utf8'?>\n<body><tag /></body>"
+        )
+
+    def test_tostring_xml_declaration_unicode_encoding(self):
+        elem = ET.XML('<body><tag/></body>')
+        preferredencoding = locale.getpreferredencoding()
+        self.assertEqual(
+            f"<?xml version='1.0' encoding='{preferredencoding}'?>\n<body><tag /></body>",
+            ET.tostring(elem, encoding='unicode', xml_declaration=True)
+        )
+
+    def test_tostring_xml_declaration_cases(self):
+        elem = ET.XML('<body><tag>ø</tag></body>')
+        preferredencoding = locale.getpreferredencoding()
+        TESTCASES = [
+        #   (expected_retval,                  encoding, xml_declaration)
+            # ... xml_declaration = None
+            (b'<body><tag>&#248;</tag></body>', None, None),
+            (b'<body><tag>\xc3\xb8</tag></body>', 'UTF-8', None),
+            (b'<body><tag>&#248;</tag></body>', 'US-ASCII', None),
+            (b"<?xml version='1.0' encoding='ISO-8859-1'?>\n"
+             b"<body><tag>\xf8</tag></body>", 'ISO-8859-1', None),
+            ('<body><tag>ø</tag></body>', 'unicode', None),
+
+            # ... xml_declaration = False
+            (b"<body><tag>&#248;</tag></body>", None, False),
+            (b"<body><tag>\xc3\xb8</tag></body>", 'UTF-8', False),
+            (b"<body><tag>&#248;</tag></body>", 'US-ASCII', False),
+            (b"<body><tag>\xf8</tag></body>", 'ISO-8859-1', False),
+            ("<body><tag>ø</tag></body>", 'unicode', False),
+
+            # ... xml_declaration = True
+            (b"<?xml version='1.0' encoding='us-ascii'?>\n"
+             b"<body><tag>&#248;</tag></body>", None, True),
+            (b"<?xml version='1.0' encoding='UTF-8'?>\n"
+             b"<body><tag>\xc3\xb8</tag></body>", 'UTF-8', True),
+            (b"<?xml version='1.0' encoding='US-ASCII'?>\n"
+             b"<body><tag>&#248;</tag></body>", 'US-ASCII', True),
+            (b"<?xml version='1.0' encoding='ISO-8859-1'?>\n"
+             b"<body><tag>\xf8</tag></body>", 'ISO-8859-1', True),
+            (f"<?xml version='1.0' encoding='{preferredencoding}'?>\n"
+             "<body><tag>ø</tag></body>", 'unicode', True),
+
+        ]
+        for expected_retval, encoding, xml_declaration in TESTCASES:
+            with self.subTest(f'encoding={encoding} '
+                              f'xml_declaration={xml_declaration}'):
+                self.assertEqual(
+                    ET.tostring(
+                        elem,
+                        encoding=encoding,
+                        xml_declaration=xml_declaration
+                    ),
+                    expected_retval
+                )
+
+    def test_tostringlist_default_namespace(self):
+        elem = ET.XML('<body xmlns="http://effbot.org/ns"><tag/></body>')
+        self.assertEqual(
+            ''.join(ET.tostringlist(elem, encoding='unicode')),
+            '<ns0:body xmlns:ns0="http://effbot.org/ns"><ns0:tag /></ns0:body>'
+        )
+        self.assertEqual(
+            ''.join(ET.tostringlist(elem, encoding='unicode', default_namespace='http://effbot.org/ns')),
+            '<body xmlns="http://effbot.org/ns"><tag /></body>'
+        )
+
+    def test_tostringlist_xml_declaration(self):
+        elem = ET.XML('<body><tag/></body>')
+        self.assertEqual(
+            ''.join(ET.tostringlist(elem, encoding='unicode')),
+            '<body><tag /></body>'
+        )
+        self.assertEqual(
+            b''.join(ET.tostringlist(elem, xml_declaration=True)),
+            b"<?xml version='1.0' encoding='us-ascii'?>\n<body><tag /></body>"
+        )
+
+        preferredencoding = locale.getpreferredencoding()
+        stringlist = ET.tostringlist(elem, encoding='unicode', xml_declaration=True)
+        self.assertEqual(
+            ''.join(stringlist),
+            f"<?xml version='1.0' encoding='{preferredencoding}'?>\n<body><tag /></body>"
+        )
+        self.assertRegex(stringlist[0], r"^<\?xml version='1.0' encoding='.+'?>")
+        self.assertEqual(['<body', '>', '<tag', ' />', '</body>'], stringlist[1:])
+
     def test_encoding(self):
         def check(encoding, body=''):
             xml = ("<?xml version='1.0' encoding='%s'?><xml>%s</xml>" %
diff --git a/Lib/xml/etree/ElementTree.py b/Lib/xml/etree/ElementTree.py
index b5ad8e1..c9e2f36 100644
--- a/Lib/xml/etree/ElementTree.py
+++ b/Lib/xml/etree/ElementTree.py
@@ -1113,6 +1113,7 @@
 # --------------------------------------------------------------------
 
 def tostring(element, encoding=None, method=None, *,
+             xml_declaration=None, default_namespace=None,
              short_empty_elements=True):
     """Generate string representation of XML element.
 
@@ -1121,13 +1122,17 @@
 
     *element* is an Element instance, *encoding* is an optional output
     encoding defaulting to US-ASCII, *method* is an optional output which can
-    be one of "xml" (default), "html", "text" or "c14n".
+    be one of "xml" (default), "html", "text" or "c14n", *default_namespace*
+    sets the default XML namespace (for "xmlns").
 
     Returns an (optionally) encoded string containing the XML data.
 
     """
     stream = io.StringIO() if encoding == 'unicode' else io.BytesIO()
-    ElementTree(element).write(stream, encoding, method=method,
+    ElementTree(element).write(stream, encoding,
+                               xml_declaration=xml_declaration,
+                               default_namespace=default_namespace,
+                               method=method,
                                short_empty_elements=short_empty_elements)
     return stream.getvalue()
 
@@ -1149,10 +1154,14 @@
         return len(self.lst)
 
 def tostringlist(element, encoding=None, method=None, *,
+                 xml_declaration=None, default_namespace=None,
                  short_empty_elements=True):
     lst = []
     stream = _ListDataStream(lst)
-    ElementTree(element).write(stream, encoding, method=method,
+    ElementTree(element).write(stream, encoding,
+                               xml_declaration=xml_declaration,
+                               default_namespace=default_namespace,
+                               method=method,
                                short_empty_elements=short_empty_elements)
     return lst
 
diff --git a/Misc/NEWS.d/next/Library/2019-03-07-20-02-18.bpo-36227.i2Z1XR.rst b/Misc/NEWS.d/next/Library/2019-03-07-20-02-18.bpo-36227.i2Z1XR.rst
new file mode 100644
index 0000000..3b5b6cd
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2019-03-07-20-02-18.bpo-36227.i2Z1XR.rst
@@ -0,0 +1,2 @@
+Added support for keyword arguments `default_namespace` and `xml_declaration` in functions
+ElementTree.tostring() and ElementTree.tostringlist().