A substantial computing system based on the Alto was developed at the Xerox Palo Alto Research Center between 1973 and 1978, and was considerably extended between 1978 and 1983. The hardware base for this system is simple and uniform, if somewhat unconventional. The software, which gives the system its visible form and knits together its many parts, is by contrast complex, variegated, even ramified. It is convenient to call the whole complex of hardware and software “the Alto system”. This paper describes the major software components of the Alto system. It also tries to explain why the system came out as it did, by tracing the ideas that influenced it, the way these ideas evolved into the concept of personal distributed computing, and the nature of the organization that built the Alto. A companion paper by Chuck Thacker describes the hardware.