From the http://download.oracle.com/javase/1.3/docs/guide/performance/hotspot.html HotSpot performance guide: The Java HotSpot Client VM has been specially tuned to reduce application start-up time and memory footprint, making it particulary well suited for client environments. The Java HotSpot Server VM is similar to the Java HotSpot Client VM, except that it has been specially tuned to maximize peak operating speed. It is intended for running long-running server applications, for which having the fastest possible operating speed is generally more important than having the fastest possible start-up time.