Extended Object Tcl (for short: XOTcl, pronounced exotickle) is an object-oriented scripting language based on Tcl. It was originally designed for providing language support for design patterns and provides novel constructs such as filters and transitive mixin classes. The language is designed for empowering rather than constraining system developers. The basic object model is highly influenced by CLOS.

XOTcl 2 is a new implementation of XOTcl based on the Next Scripting Framework and is highly compatible with XOTcl 1. Most XOTcl 1 scripts run unmodified under XOTcl 2. Large code bases running XOTcl, such as OpenACS  with xotcl-core and xowiki, run with just a few lines of compatibility layer. The detailed incompatibilities are described in the Next Migration Guide .