This user manual describes the Arc Habitat Suitability Index (ArcHSI), which is a geographical information system (GIS) model that estimates the ability of an area to meet the food and cover requirements of an animal species. The components and parameters of the model occur in tables and can be easily edited or otherwise modified. ArcHSI runs on personal computers with the full installation of ArcGIS (Version 8.2+). ArcHSI is a tool intended to inform planners of the probable impacts on wildlife for the alternatives they develop, and it augments, rather than supplants, the expertise of resource specialists with site-specific knowledge. System requirements are platform specific, but a typical system requires 512 megabytes RAM and a 1 gHz CPU. A Unix ArcINFO version is also available. We have included a demonstration (see DEMONSTRATION) that provides a step-by-step tour of the software, including setting up the first run; interpreting, formatting, and analyzing output; and identifying and solving the most common problems experienced by new users.