In 1989 Ralph Butler and Ewing Lusk began a complete rewrite of the entire system, with the goal of producing a very robust system for wide distribution that included features present from the beginning as well as new features (message passing among heterogeneous machines, global operations) that users were requesting. There were also a great variety of new parallel machines and workstations to support. This effort, concentrated in 1990 and 1991, produced the current p4, which takes its name from the title of the 1987 book [4] and its functionality from all previous versions, going all the way back to the HEP. The interface has changed little since p4 was first released, although many performance improvements have been made, and programs written for the p4 of 1989 run essentially unchanged on machines only released in 1993. While a number of features have been added, little has been removed, and virtually all p4 programs ever written will run on the current version (which, as this is being written in April of 1993, is version 1.3).