diff --git a/redo/basicctrls_windows.c b/redo/basicctrls_windows.c
index 1676776..c82c5d6 100644
--- a/redo/basicctrls_windows.c
+++ b/redo/basicctrls_windows.c
@@ -104,3 +104,23 @@ void setTextFieldSubclass(HWND hwnd, void *data)
 	if ((*fv_SetWindowSubclass)(hwnd, textfieldSubProc, 0, (DWORD_PTR) data) == FALSE)
 		xpanic("error subclassing TextField to give it its own event handler", GetLastError());
 }
+
+void textfieldSetAndShowInvalidBalloonTip(HWND hwnd, WCHAR *text)
+{
+	EDITBALLOONTIP ti;
+
+	ZeroMemory(&ti, sizeof (EDITBALLOONTIP));
+	ti.cbStruct = sizeof (EDITBALLOONTIP);
+	ti.pszTitle = L"Invalid Input";		// TODO verify
+	ti.pszText = text;
+	ti.ttiIcon = TTI_ERROR;
+	if (SendMessageW(hwnd, EM_SHOWBALLOONTIP, 0, (LPARAM) (&ti)) == FALSE)
+		xpanic("error showing TextField.Invalid() balloon tip", GetLastError());
+	MessageBeep(0xFFFFFFFF);		// TODO can this return an error?
+}
+
+void textfieldHideInvalidBalloonTip(HWND hwnd)
+{
+	if (SendMessageW(hwnd, EM_HIDEBALLOONTIP, 0, 0) == FALSE)
+		xpanic("error hiding TextField.Invalid() balloon tip", GetLastError());
+}
diff --git a/redo/textfield_unix.go b/redo/textfield_unix.go
index e9624ee..cef2765 100644
--- a/redo/textfield_unix.go
+++ b/redo/textfield_unix.go
@@ -71,6 +71,7 @@ func (t *textfield) Invalid(reason string) {
 	creason := togstr(reason)
 	defer freegstr(creason)
 	C.gtk_entry_set_icon_tooltip_text(t.entry, C.GTK_ENTRY_ICON_SECONDARY, creason)
+	// TODO beep
 }
 
 //export textfieldChanged
diff --git a/redo/textfield_windows.go b/redo/textfield_windows.go
index 7345205..5f89691 100644
--- a/redo/textfield_windows.go
+++ b/redo/textfield_windows.go
@@ -51,7 +51,11 @@ func (t *textfield) OnChanged(f func()) {
 }
 
 func (t *textfield) Invalid(reason string) {
-	// TODO
+	if reason == "" {
+		C.textfieldHideInvalidBalloonTip(t._hwnd)
+		return
+	}
+	C.textfieldSetAndShowInvalidBalloonTip(t._hwnd, toUTF16(reason))
 }
 
 //export textfieldChanged
diff --git a/redo/winapi_windows.h b/redo/winapi_windows.h
index ea9fe17..5787331 100644
--- a/redo/winapi_windows.h
+++ b/redo/winapi_windows.h
@@ -64,6 +64,8 @@ extern void setCheckboxSubclass(HWND, void *);
 extern BOOL checkboxChecked(HWND);
 extern void checkboxSetChecked(HWND, BOOL);
 extern void setTextFieldSubclass(HWND, void *);
+extern void textfieldSetAndShowInvalidBalloonTip(HWND, WCHAR *);
+extern void textfieldHideInvalidBalloonTip(HWND);
 
 // init_windows.c
 extern HINSTANCE hInstance;