SWI-Prolog is a comprehensive and portable implementation of the Prolog programming language. SWI-Prolog aims to be a robust and scalable implementation supporting a wide range of applications. In particular, it ships with a wide range of interface libraries, providing interfaces to other languages, databases, graphics and networking. It provides extensive support for managing HTML/SGML/XML and RDF documents. The system is particularly suited for server applications due to robust support for multi threading and HTTP server libraries. SWI-Prolog is designed in the `Edinburgh tradition'. In addition to the ISO Prolog standard it is largely compatible to Quintus, SICStus and YAP Prolog. SWI-Prolog provides a compatibility framework developed in cooperation with YAP and instantiated for YAP, SICStus and IF/Prolog. SWI-Prolog aims at providing a good development environment, including extensive editor support, graphical source-level debugger, autoloading and `make' facility and much more. SWI-Prolog editor and the PDT plugin for Eclipse provide alternative environments. This document gives an overview of the features, system limits and built-in predicates.