<p> The Unimak Island caribou herd is in the midst of a population decline associated with low birth rates and calf survival. I evaluated the spatial and temporal complexity of caribou landscape use and the availability and nutritional quality of key forage species. First, I examined seasonal landscape use and movement using GPS collar data. Second, I determined seasonal diets to the species-level for spring, summer, and fall using a novel n-alkane and long-chain fatty acid (LCOH) method. Finally, I compared diet quality with needs for maintenance and to the quality of available plant species. Caribou home ranges were largest and movements least in the winter, and movements appeared dependent on plant phenology and weather conditions. Diets were complex and seasonally variable but generally contained higher proportions of forbs than other caribou herds likely due to their higher relative availability as compared to other preferred forages. Finally, animals foraged selectively each season, obtaining a diet that was higher in digestible dry matter and nitrogen than the average of plants available. Nutritional quality appeared to be well above requirements during the three seasons investigated. Overall, I concluded that spring, summer, and fall forage quality and availability do not impose strong constraints on Unimak Island caribou. However, I recommend that diet composition and quality be determined in the winter, the time when forage often limits the productivity of caribou herds.</p>