diff --git a/coverage/ctracer/tracer.c b/coverage/ctracer/tracer.c
index c05dff643..ed05a42d0 100644
--- a/coverage/ctracer/tracer.c
+++ b/coverage/ctracer/tracer.c
@@ -818,7 +818,7 @@ CTracer_trace(CTracer *self, PyFrameObject *frame, int what, PyObject *arg_unuse
     return RET_OK;
     #endif
 
-    if (!self->started) {
+    if (!atomic_load(&self->started)) {
         /* If CTracer.stop() has been called from another thread, the tracer
            is still active in the current thread. Let's deactivate ourselves
            now. */
@@ -848,7 +848,7 @@ CTracer_trace(CTracer *self, PyFrameObject *frame, int what, PyObject *arg_unuse
     Py_DECREF(ascii);
     #endif
 
-    self->activity = TRUE;
+    atomic_store(&self->activity, TRUE);
 
     switch (what) {
     case PyTrace_CALL:
@@ -981,9 +981,10 @@ CTracer_call(CTracer *self, PyObject *args, PyObject *kwds)
 static PyObject *
 CTracer_start(CTracer *self, PyObject *args_unused)
 {
+    assert(atomic_load(&self->started) == FALSE);
     PyEval_SetTrace((Py_tracefunc)CTracer_trace, (PyObject*)self);
-    self->started = TRUE;
     self->tracing_arcs = self->trace_arcs && PyObject_IsTrue(self->trace_arcs);
+    atomic_store(&self->started, TRUE);
 
     /* start() returns a trace function usable with sys.settrace() */
     Py_INCREF(self);
@@ -993,13 +994,11 @@ CTracer_start(CTracer *self, PyObject *args_unused)
 static PyObject *
 CTracer_stop(CTracer *self, PyObject *args_unused)
 {
-    if (self->started) {
-        /* Set the started flag only. The actual call to
-           PyEval_SetTrace(NULL, NULL) is delegated to the callback
-           itself to ensure that it called from the right thread.
-           */
-        self->started = FALSE;
-    }
+    /* Set the started flag only. The actual call to
+       PyEval_SetTrace(NULL, NULL) is delegated to the callback
+       itself to ensure that it called from the right thread.
+       */
+    atomic_store(&self->started, FALSE);
 
     Py_RETURN_NONE;
 }
@@ -1007,7 +1006,7 @@ CTracer_stop(CTracer *self, PyObject *args_unused)
 static PyObject *
 CTracer_activity(CTracer *self, PyObject *args_unused)
 {
-    if (self->activity) {
+    if (atomic_load(&self->activity)) {
         Py_RETURN_TRUE;
     }
     else {
@@ -1018,7 +1017,7 @@ CTracer_activity(CTracer *self, PyObject *args_unused)
 static PyObject *
 CTracer_reset_activity(CTracer *self, PyObject *args_unused)
 {
-    self->activity = FALSE;
+    atomic_store(&self->activity, FALSE);
     Py_RETURN_NONE;
 }
 
diff --git a/coverage/ctracer/tracer.h b/coverage/ctracer/tracer.h
index b00134de9..7f3f85679 100644
--- a/coverage/ctracer/tracer.h
+++ b/coverage/ctracer/tracer.h
@@ -32,11 +32,11 @@ typedef struct CTracer {
     PyObject * disable_plugin;
 
     /* Has the tracer been started? */
-    BOOL started;
+    _Atomic BOOL started;
     /* Are we tracing arcs, or just lines? */
     BOOL tracing_arcs;
     /* Have we had any activity? */
-    BOOL activity;
+    _Atomic BOOL activity;
     /* The current dynamic context. */
     PyObject * context;
 
diff --git a/coverage/ctracer/util.h b/coverage/ctracer/util.h
index 94591e0d2..cef8e0c4d 100644
--- a/coverage/ctracer/util.h
+++ b/coverage/ctracer/util.h
@@ -5,6 +5,7 @@
 #define _COVERAGE_UTIL_H
 
 #include <Python.h>
+#include <stdatomic.h>
 
 /* Compile-time debugging helpers */
 #undef WHAT_LOG         /* Define to log the WHAT params in the trace function. */
diff --git a/setup.py b/setup.py
index 92fad6502..45c501adf 100644
--- a/setup.py
+++ b/setup.py
@@ -155,6 +155,11 @@ def run(self):
 
     def build_extension(self, ext):
         """Wrap `build_extension` with `BuildFailed`."""
+        if self.compiler.compiler_type == "msvc":
+            ext.extra_compile_args = (ext.extra_compile_args or []) + [
+                "/std:c11",
+                "/experimental:c11atomics",
+            ]
         try:
             # Uncomment to test compile failure handling:
             #   raise errors.CCompilerError("OOPS")