The Java Plug-In is a very important tool provided by Sun, however it is not nearly as well-known or widely used as the Java SDK. The purpose of the Java Plug-In is to provide a certified Java runtime environment for applets running within Web browsers. At the time of this writing, Netscape Navigator 4.5 and Microsoft Internet Explorer 5.0 both included Java support. However, this support is buggy at best and downright lacking in some areas in both Web browsers. Developers have found that it is extremely difficult to get any Java applet of even moderate complexity to work correctly in both these browsers due to JVM problems with both products. Meanwhile, however, both browsers support a "plug-in" model that works very well.