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, JSON, YAML and RDF documents. The system is particularly suited for server applications due to robust support for multithreading and HTTP server libraries.

SWI-Prolog extends Prolog with tabling (SGL resolution). Tabling provides better termination properties and avoids repetitive recomputation. Following XSB, SWI-Prolog's tabling supports sound negation using the Well Founded Semantics. Incremental tabling supports usage as a Deductive database.

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, IF/Prolog and XSB.

SWI-Prolog aims at providing a rich development environment, including extensive editor support, graphical source-level debugger, autoloading, a‘make' facility to reload edited files and much more. GNU-Emacs, SWI-Prolog editor for Windows, the PDT plugin for Eclipse or a Visual Studio Code plugin provide alternative environments. SWISH provides a web based environment.

This document gives an overview of the features, system limits and built-in predicates.