diff --git a/flit/validate.py b/flit/validate.py
index d278fa01..dd388b14 100644
--- a/flit/validate.py
+++ b/flit/validate.py
@@ -134,7 +134,8 @@ def _is_identifier_attr(s):
          \s*(?P<version>[(=~<>!][^;]*)?
          \s*(?P<envmark>;.*)?
      $""", re.IGNORECASE | re.VERBOSE)
-MARKER_OP = re.compile(r'(~=|===?|!=|<=?|>=?|\s+in\s+|\s+not in\s+)')
+MARKER_OP_PATTERN = r'(?:~=|===?|!=|<=?|>=?|\s+in\s+|\s+not in\s+)'
+MARKER_OP = re.compile(MARKER_OP_PATTERN)
 
 def validate_name(metadata):
     name = metadata.get('name', None)
@@ -163,21 +164,37 @@ def validate_requires_python(metadata):
 }
 
 def validate_environment_marker(em):
-    clauses = re.split(r'\s+(?:and|or)\s+', em)
     problems = []
-    for c in clauses:
-        # TODO: validate parentheses properly. They're allowed by PEP 508.
-        parts = MARKER_OP.split(c.strip('()'))
-        if len(parts) != 3:
-            problems.append("Invalid expression in environment marker: {!r}".format(c))
+    marker_var_ops_value_pattern = r"""\b\S+\b """ + MARKER_OP_PATTERN + r""" (?:"\b\S+\b"|'\b\S+\b')"""
+
+    # check pattern "VAR OPS VALUE"
+    var_value_pair = re.findall(marker_var_ops_value_pattern, em)
+    for var_value in var_value_pair:
+        parts = re.compile(MARKER_OP_PATTERN).split(var_value)
+        if len(parts) != 2:
+            problems.append("Invalid expression in environment marker")
             continue
-        l, op, r = parts
-        for var in (l.strip(), r.strip()):
-            if var[:1] in {'"', "'"}:
-                if len(var) < 2 or var[-1:] != var[:1]:
-                    problems.append("Invalid string in environment marker: {}".format(var))
-            elif var not in MARKER_VARS:
-                problems.append("Invalid variable name in environment marker: {!r}".format(var))
+        l, r = parts
+        if l.strip() not in MARKER_VARS:
+            problems.append("Invalid variable name in environment marker: {!r}".format(l))
+
+    # check grammars and parentheses
+    expressions_str = re.sub(marker_var_ops_value_pattern, '$', em)
+    prev_exp_str = expressions_str.replace(" ", "")
+    done_reduced = False
+    curr_exp_str = ""
+    while not done_reduced:  # reduce parse tree
+        curr_exp_str = prev_exp_str \
+            .replace("$and$", "$") \
+            .replace("$or$", "$") \
+            .replace("($)", "$")
+        if len(prev_exp_str) == len(curr_exp_str): # no change so reduce is done
+            done_reduced = True
+        prev_exp_str = curr_exp_str
+    if '(' in curr_exp_str or ')' in curr_exp_str:
+        problems.append("parentheses are not balanced or occur in the incorrect places")
+    if curr_exp_str != "$":
+        problems.append("environment marker syntax is invalid")
     return problems
 
 def validate_requires_dist(metadata):
diff --git a/tests/test_validate.py b/tests/test_validate.py
index be2b7ad6..a35f62de 100644
--- a/tests/test_validate.py
+++ b/tests/test_validate.py
@@ -60,9 +60,11 @@ def test_validate_environment_marker():
 
     assert vem('python_version >= "3" and os_name == \'posix\'') == []
 
+    assert vem("""extra == "test" and (os_name == "nt" or python_version == "2.7")""") == []
+
     res = vem('python_version >= "3')  # Unclosed string
     assert len(res) == 1
-    assert res[0].startswith("Invalid string")
+    assert res[0] == "environment marker syntax is invalid"
 
     res = vem('python_verson >= "3"')  # Misspelled name
     assert len(res) == 1
@@ -70,13 +72,23 @@ def test_validate_environment_marker():
 
     res = vem("os_name is 'posix'")  # No 'is' comparisons
     assert len(res) == 1
-    assert res[0].startswith("Invalid expression")
+    assert res[0] == "environment marker syntax is invalid"
 
     res = vem("'2' < python_version < '4'")  # No chained comparisons
     assert len(res) == 1
-    assert res[0].startswith("Invalid expression")
+    assert res[0] == "environment marker syntax is invalid"
+
+    res = vem("(python_version < '4'))")  # No chained comparisons
+    assert len(res) == 2
+    assert "parentheses are not balanced or occur in the incorrect places" in res
+
+    res = vem("((python_version < '4') or (python_version < '8')")  # No chained comparisons
+    assert len(res) == 2
+    assert "parentheses are not balanced or occur in the incorrect places" in res
+
+    assert len(vem('(wrongMarkerVar < \'4\'))')) == 3
 
-    assert len(vem('os.name == "linux\'')) == 2
+    assert len(vem('wrongMarkerVar == "linux" and wrongMarkerVar2 == \'some_value\'')) == 2
 
 def test_validate_url():
     vurl = fv.validate_url