VM/370 is an operating system which provides its multiple users with seemingly separate and independent IBM System/370 computing systems. These virtual machines are simulated using IBM System/370 hardware and have its same architecture. In addition, VM/370 provides a single-user interactive system for personal computing and a computer network system for information interchange among interconnected machines. VM/370 evolved from an experimental operating system designed and built over fifteen years ago. This paper reviews the historical environment, design influences, and goals which shaped the original system.