The chemical composition and nutritional quality of grains and seeds do not change substantially during storage; that is, insects that feed on stored grain have stable food source, without major changes in nutritional composition and their defense compounds over time. Despite the nutritional requirements of insects feeding on stored products being similar to those of other phytophagous species, the former exhibit an almost unique ability to grow and reproduce on relatively dry food. Early studies indicated that these insects use metabolic water for their development in such a dry environment (Fraenkel and Blewett 1944; Baker and Loschiavo 1987). Other studies, however, show that the passive diffusion of water vapor is also an important source of water for stored-product insects (Arlian 1979; Arlian and Veselica 1979). The wide availability of food coupled with adequate temperature and relative humidity (RH) favor population growth and distribution of these insects. This chapter explores the physiological18.1 Introduction 417 18.2 Grain Storage and Losses 418 18.3 Storage Ecosystem 418 18.4 Major Stored Grain Pests: Feeding Habits and Damage Caused 420 18.5 Mouthparts, Digestive and Excretory Systems 424 18.6 Food and Nutrition Characteristics 425 18.7 Search for Food and Its Utilization 42618.7.1 Stimuli for Oviposition 426 18.7.2 Food Attractants and Gustatory Stimuli 427 18.7.3 Nutritional Requirements 427 18.7.4 Digestive Enzymes 430 18.7.5 Nutrient Budget and Relative Growth Rate 432 18.7.6 Microorganisms 43418.8 Physiological and Behavioral Adaptations to Food and Environmental Changes 434 18.9 Applications and Perspectives for Stored Pest Management 435