The field of Performance and Capacity is a cross between Operating Systems and Application analysis and programming on one side, and mathematical statistics on the other. A few years ago Performance and Capacity was mainly a mainframe related job performed by MVS gurus. It entered the UNIX servers world in the 1991 - 1992 time frame, and became very visible with the emergence of WEB related applications servers. While in the MVS world it was expensive to waste system resources, in the WEB world it's expensive not having enough resources to sustain a competitive response time.