diff --git a/configure.ac b/configure.ac
index c8403c816..e5dfc5327 100644
--- a/configure.ac
+++ b/configure.ac
@@ -190,6 +190,7 @@ AC_SUBST(CFG_CXXFLAGS_STD_OLDEST)
 _MY_CXX_CHECK_OPT(CFG_CXXFLAGS_WEXTRA,-Wextra)
 _MY_CXX_CHECK_OPT(CFG_CXXFLAGS_WEXTRA,-Wfloat-conversion)
 _MY_CXX_CHECK_OPT(CFG_CXXFLAGS_WEXTRA,-Wlogical-op)
+_MY_CXX_CHECK_OPT(CFG_CXXFLAGS_WEXTRA,-Wthread-safety)
 AC_SUBST(CFG_CXXFLAGS_WEXTRA)
 
 # Flags for compiling Verilator internals including parser always
diff --git a/include/verilated.mk.in b/include/verilated.mk.in
index 584427299..6eec3a240 100644
--- a/include/verilated.mk.in
+++ b/include/verilated.mk.in
@@ -109,6 +109,14 @@ ifeq ($(VM_SC),1)
  endif
 endif
 
+#######################################################################
+##### Threaded builds
+
+ifneq ($(VM_THREADS),0)
+  # Need C++11 at least, so always default to newest
+  CPPFLAGS += -DVM_THREADED $(CFG_CXXFLAGS_STD_NEWEST)
+endif
+
 #######################################################################
 ##### Stub
 
diff --git a/src/V3EmitMk.cpp b/src/V3EmitMk.cpp
index ad8e9346e..6eeb008d8 100644
--- a/src/V3EmitMk.cpp
+++ b/src/V3EmitMk.cpp
@@ -62,6 +62,8 @@ public:
 	of.puts("\n### Switches...\n");
 	of.puts("# Coverage output mode?  0/1 (from --coverage)\n");
 	of.puts("VM_COVERAGE = "); of.puts(v3Global.opt.coverage()?"1":"0"); of.puts("\n");
+	of.puts("# Threaded output mode?  0/1/N threads (from --threads)\n");
+	of.puts("VM_THREADS = "); of.puts(cvtToStr(v3Global.opt.threads())); of.puts("\n");
 	of.puts("# Tracing output mode?  0/1 (from --trace)\n");
 	of.puts("VM_TRACE = "); of.puts(v3Global.opt.trace()?"1":"0"); of.puts("\n");
 
diff --git a/src/V3Options.cpp b/src/V3Options.cpp
index 911dc60e9..cfb0b7ab2 100644
--- a/src/V3Options.cpp
+++ b/src/V3Options.cpp
@@ -984,6 +984,11 @@ void V3Options::parseOptsList(FileLine* fl, const string& optdir, int argc, char
 		shift; m_prefix = argv[i];
 		if (m_modPrefix=="") m_modPrefix = m_prefix;
 	    }
+	    else if ( !strcmp (sw, "-no-threads") ) { m_threads = 0; }  // Undocumented until functional
+	    else if ( !strcmp (sw, "-threads") && (i+1)<argc ) {  // Undocumented until functional
+		shift; m_threads = atoi(argv[i]);
+		if (m_threads < 0) fl->v3fatal("--threads must be >= 0: "<<argv[i]);
+	    }
 	    else if ( !strcmp (sw, "-top-module") && (i+1)<argc ) {
 		shift; m_topModule = argv[i];
 	    }
@@ -1218,6 +1223,7 @@ V3Options::V3Options() {
     m_stats = false;
     m_statsVars = false;
     m_systemC = false;
+    m_threads = 0;
     m_trace = false;
     m_traceDups = false;
     m_traceParams = true;
diff --git a/src/V3Options.h b/src/V3Options.h
index 1f3a71547..acd8e964d 100644
--- a/src/V3Options.h
+++ b/src/V3Options.h
@@ -113,6 +113,7 @@ class V3Options {
     int		m_outputSplitCFuncs;// main switch: --output-split-cfuncs
     int		m_outputSplitCTrace;// main switch: --output-split-ctrace
     int		m_pinsBv;	// main switch: --pins-bv
+    int		m_threads;	// main switch: --threads (0 == --no-threads)
     int		m_traceDepth;	// main switch: --trace-depth
     int		m_traceMaxArray;// main switch: --trace-max-array
     int		m_traceMaxWidth;// main switch: --trace-max-width
@@ -258,6 +259,7 @@ class V3Options {
     int	   outputSplitCFuncs() const { return m_outputSplitCFuncs; }
     int	   outputSplitCTrace() const { return m_outputSplitCTrace; }
     int	   pinsBv() const { return m_pinsBv; }
+    int	   threads() const { return m_threads; }
     int	   traceDepth() const { return m_traceDepth; }
     int	   traceMaxArray() const { return m_traceMaxArray; }
     int	   traceMaxWidth() const { return m_traceMaxWidth; }
diff --git a/test_regress/Makefile_obj b/test_regress/Makefile_obj
index 441b5a9ea..c6bdd069b 100644
--- a/test_regress/Makefile_obj
+++ b/test_regress/Makefile_obj
@@ -40,8 +40,6 @@ ifeq ($(CFG_WITH_LONGTESTS),yes)
     CPPFLAGS += $(CFG_CXXFLAGS_STD_NEWEST)
   else ifeq ($(DRIVER_STD),oldest)
     CPPFLAGS += $(CFG_CXXFLAGS_STD_OLDEST)
-  else ifneq ($(findstring VL_THREADED,$(CPPFLAGS)),)
-    CPPFLAGS += $(CFG_CXXFLAGS_STD_NEWEST)
   endif
 endif
 
diff --git a/test_regress/t/t_include_all_newest.pl b/test_regress/t/t_include_all_newest.pl
index b386a4e71..dbe936ef9 100755
--- a/test_regress/t/t_include_all_newest.pl
+++ b/test_regress/t/t_include_all_newest.pl
@@ -13,8 +13,7 @@ my $root = "..";
 
 compile (
     # Can't use --coverage and --savable together, so cheat and compile inline
-    verilator_flags2 => ['--cc --coverage-toggle --coverage-line --coverage-user --trace --vpi $root/include/verilated_save.cpp'],
-    make_flags => 'DRIVER_STD=newest CPPFLAGS_ADD=-DVL_THREADED=1',
+    verilator_flags2 => ['--cc --coverage-toggle --coverage-line --coverage-user --trace --threads 1 --vpi $root/include/verilated_save.cpp'],
     );
 
 execute (
diff --git a/test_regress/t/t_threads_counter.v b/test_regress/t/t_threads_counter.v
new file mode 100644
index 000000000..dabcf0a77
--- /dev/null
+++ b/test_regress/t/t_threads_counter.v
@@ -0,0 +1,24 @@
+// DESCRIPTION: Verilator: Verilog Test module
+//
+// This file ONLY is placed into the Public Domain, for any use,
+// without warranty, 2017 by Wilson Snyder.
+
+module t (/*AUTOARG*/
+   // Inputs
+   clk
+   );
+
+   input clk;
+
+   integer cyc;
+
+   always @ (posedge clk) begin
+      cyc <= cyc + 1;
+      if (cyc!=0) begin
+         if (cyc==10) begin
+            $write("*-* All Finished *-*\n");
+            $finish;
+         end
+      end
+   end
+endmodule
diff --git a/test_regress/t/t_threads_counter_0.pl b/test_regress/t/t_threads_counter_0.pl
new file mode 100755
index 000000000..3bcf004fd
--- /dev/null
+++ b/test_regress/t/t_threads_counter_0.pl
@@ -0,0 +1,21 @@
+#!/usr/bin/perl
+if (!$::Driver) { use FindBin; exec("$FindBin::Bin/bootstrap.pl", @ARGV, $0); die; }
+# DESCRIPTION: Verilator: Verilog Test driver/expect definition
+#
+# Copyright 2003-2009 by Wilson Snyder. This program is free software; you can
+# redistribute it and/or modify it under the terms of either the GNU
+# Lesser General Public License Version 3 or the Perl Artistic License
+# Version 2.0.
+
+top_filename("t/t_threads_counter.v");
+
+compile (
+    verilator_flags2 => ['--cc --no-threads'],
+    );
+
+execute (
+    check_finished=>1,
+    );
+
+ok(1);
+1;
diff --git a/test_regress/t/t_threads_counter_1.pl b/test_regress/t/t_threads_counter_1.pl
new file mode 100755
index 000000000..8cf80808b
--- /dev/null
+++ b/test_regress/t/t_threads_counter_1.pl
@@ -0,0 +1,21 @@
+#!/usr/bin/perl
+if (!$::Driver) { use FindBin; exec("$FindBin::Bin/bootstrap.pl", @ARGV, $0); die; }
+# DESCRIPTION: Verilator: Verilog Test driver/expect definition
+#
+# Copyright 2003-2009 by Wilson Snyder. This program is free software; you can
+# redistribute it and/or modify it under the terms of either the GNU
+# Lesser General Public License Version 3 or the Perl Artistic License
+# Version 2.0.
+
+top_filename("t/t_threads_counter.v");
+
+compile (
+    verilator_flags2 => ['--cc --threads 1'],
+    );
+
+execute (
+    check_finished=>1,
+    );
+
+ok(1);
+1;
diff --git a/test_regress/t/t_threads_counter_2.pl b/test_regress/t/t_threads_counter_2.pl
new file mode 100755
index 000000000..c1519604c
--- /dev/null
+++ b/test_regress/t/t_threads_counter_2.pl
@@ -0,0 +1,21 @@
+#!/usr/bin/perl
+if (!$::Driver) { use FindBin; exec("$FindBin::Bin/bootstrap.pl", @ARGV, $0); die; }
+# DESCRIPTION: Verilator: Verilog Test driver/expect definition
+#
+# Copyright 2003-2009 by Wilson Snyder. This program is free software; you can
+# redistribute it and/or modify it under the terms of either the GNU
+# Lesser General Public License Version 3 or the Perl Artistic License
+# Version 2.0.
+
+top_filename("t/t_threads_counter.v");
+
+compile (
+    verilator_flags2 => ['--cc --threads 2'],
+    );
+
+execute (
+    check_finished=>1,
+    );
+
+ok(1);
+1;