I'm not sure why I get a java.lang.NoClassDefFoundError runtime error on the following line: this.startBtn.addMouseListener(new MouseAdapter() { public void mouseClicked(MouseEvent e) { startClicked(); } }); in an Applet. It compiles fine. How can the jre not find a core awt class? I use the same line in a different applet without issues. What am I not understanding? Thanks in advance