1. Introduction

XSB is a research-oriented, commercial-grade Logic Programming system for Unix and Windows-based platforms. In addition to providing nearly all functionality of ISO-Prolog, XSB includes the following features:

• Evaluation of queries according to the Well-Founded Semantics [57] through full SLG resolution;
• A fully multi-threaded evaluation mechanism with thread-shared static code, and that allows dynamic code and tables to be thread-shared or thread-private.
• Constraint handling for tabled programs based on an engine-level implementation of annotated variables and a package, clpqr for handling real constraints, and the addition of Constraint Handling Rules ;
• A variety of indexing techniques for asserted code including variable-depth indexing on several alternate arguments, fixed-depth indexing on combined arguments, trie-indexing. In addition, both backtrackable and non-backtrackable updates to asserted code are supported.
• A set of mature packages, to extend XSB to evaluate F-logic [34] through the FLORA package, to model check concurrent systems through the XMC system, to manage ontologies through the Cold Dead Fish package, to support literate programming through the xsbdoc package, and to support answer set programming through the XASP package among other features.
• A number of interfaces to other software systems, such a C, Java, Perl, ODBC, SModels [43], and Oracle.
• Fast loading of large files by the load_dync predicate, and by other means.
• A compiled HiLog implementation;
• Backtrackable updates through XSB's storage module that support the semantics of transaction logic [6].
• Extensive pattern matching packages, and interfaces to libwww routines, all of which are especially useful for Web applications.
• A novel transformation technique called unification factoring that can improve program speed and indexing for compiled code;
• Macro substitution for Prolog files via the xpp preprocessor (included with the XSB distribution).
• Preprocessors and Interpreters so that XSB can be used to evaluate programs that are based on advanced formalisms, such as extended logic programs (according to the Well-Founded Semantics [2]); Generalized Annotated Programs [35].
• Source code availability for portability and extensibility under the GNU General Public Library License.

Though XSB can be used as a Prolog system^1.1, we avoid referring to XSB as such, because of the availability of SLG resolution and the handling of HiLog terms. These facilities, while seemingly simple, significantly extend its capabilities beyond those of a typical Prolog system. We feel that these capabilities justify viewing XSB as a new paradigm for Logic Programming. We briefly discuss some of these features; others are discussed in Volumes 1 and 2 of the XSB manual, as well as the manuals for various XSB packages such as FLORA, XMC, Cold Dead Fish, xsbdoc, and XASP.