Title: Expanding the MapWindow GIS 6 Modeler Toolset MapWindow GIS is a free and open source desktop GIS used by GIS analysts, technicians, and programmers worldwide. With over 6000 downloads per month from the MapWindow.org web site, this software is the most widely used open source GIS for Windows. With the introduction of version 6, the MapWindow project adds an integrated graphical modeling environment suitable for linking together independent geospatial and temporal processes at a granular level. MapWindow Modeler relies on an extensible modeling environment framework, which allows users to easily create models that can take advantage of spatial and temporal data objects and analytical tools. One of the central design considerations of MapWindow GIS version 6 was to keep the entire project as modular as possible. Breaking components down into modular elements simplifies integration with other software projects and increases the ability to update, improve, and add to the MapWindow software. The MapWindow Modeler uses this same componentized approach to make it relatively simple to take existing modeling processes, wrap them in an appropriate interface, and execute them as part of a more complex model. The set of tools available within MapWindow Modeler is growing to include built-in tools based on basic GIS functionality (projections, overlay, etc.) Additionally, using the modular and extensible design paradigm, the system has expanded to include tools from the open source GRASS library and from the open source Sextante library. It is also possible to link the modeler with tools provided by Web Processing Services, which remotely process geospatial information and return the results to the modeler. MapWindow GIS 6 and the MapWindow Modeler are entirely developed using the Microsoft .NET Framework which allows it to be run Windows, Linux or OS X (via the Mono compiler). This presentation will briefly introduce the MapWindow GIS project and open source community, then provide a detailed overview of the use of the MapWindow Modeler with application to some specific spatio-temporal modeling problems.