You've probably heard: OS X 10.5 (Leopard) runs Java exceptionally slowly if at all. This is because they've turned off OpenGL hardware acceleration (or what little of it they had in 10.4). I've been told that a number of Java simulation environments, such as Processing, are having difficulty. I have not tested MASON under Leopard and cannot verify that it will run well or not. But given Leopard's serious Java problems, you may wish delaying any upgrade until a later version (10.5.1, perhaps). Sean