This Manual corresponds to SICStus Prolog release 2.1. #8 Prolog is a simple but powerful programming language developed at the University of Marseilles (Prolog : Manuel de Reference et d'Utilisation by P.Roussel, Groupe d'Intelligence Artificielle, Marseille-Luminy, 1975), as a practical tool for programming in Logic (Logic for Problem Solving by R.A. Kowalski, DCL Memo 75, Dept. of Artificial Intelligence, University of Edinburgh, March, 1974.)) From a user's point of view the major attraction of the language is ease of programming. Clear, readable, concise programs can be written quickly with few errors. This manual describes a Prolog system developed at the Swedish Institute of Computer Science in collaboration with Ericsson Telecom AB, NobelTech Systems AB, Infologics AB and Televerket under the IT4 program. The system consists of a WAM emulator written in C, a library and runtime system written in C and Prolog and an interpreter and a compiler written in Prolog. The Prolog engine is a Warren Abstract Machine (WAM) emulator defined by D:H:D: Warren in An Abstract Prolog Instruction Set, Tech. Note 309, International, Menlo Park, CA, 1983. Two modes of compilation are available: in-core i.e. incremental, and file-to-file.