We describe a food-based system for quantitatively evaluating habitat quality for deer called the Forage Resource Evaluation System for Habitat and provide its rationale and suggestions for use. The system was developed as a tool for wildlife biologists and other natural resource managers and planners interested in evaluating habitat quality and, especially, comparing two or more patches of habitat or the same patch at different seasons or under different conditions. It is based on the quantity (of biomass) and quality (digestible energy and digestible protein) of the habitat's food resources in relation to user-specified metabolic requirements of deer (which differ with species, age, sex, season, and reproductive status). It uses a linear programming algorithm to determine the suitable forage that can sustain deer at the specified requirements. Output includes the number of deer days (1 deer day equals one deer for 1 day) per unit area that the available food resources are capable of supporting, the species composition of the solution set to the linear programming problem, and the relative importance of biomass versus nutritional quality as limiting factors of the habitat for deer. The system is accessed via the Internet (http://cervid.uaa.alaska.edu/deer/home.aspx) and consists of a Web-based application for analysis at the patch (or "stand") scale and a geographical information system (GIS)-based application for analysis at the landscape scale, which includes spatial effects of patch sizes and their shapes and locations in relation to deer home ranges. Although the system was developed for Sitka black-tailed deer (Odocoileus hemionus sitkensis) in southeastern Alaska and illustrated with examples for them, it also can be applied for other species of deer (with the exception of very large species such as moose, Alces alces) elsewhere in the world.