Roscoe is a multi-computer operating system running on a network of LSI-11 computers at the University of Wisconsin. This document describes Roscoe from the viewpoint of a user or a writer of user-level programs. All system service calls and library routines are described in detail. In addition, the command-line interpreter and terminal input conventions are discussed. Companion reports describe the purposes and concepts underlying the Roscoe project and give detailed accounts of the Roscoe utility kernel and utility processes.