The chapter discusses the evolution and maintenance of host specialization in phytophagous insects, focusing on the factors that influence their diet breadth. Insects, being the most diverse group of organisms, have a wide range of host plants, and their host preferences are influenced by plant chemistry, ecological variables, and physiological states. The text outlines various models of host specialization, including optimality models, population genetic models, and physiological state models, which predict different behaviors and preferences based on factors such as egg load, search time, and plant quality. Proximate causes of intraspecific variation in host use are discussed, emphasizing the roles of learning, physiological state, and genetic variation. Learning can affect host acceptance, while physiological state influences the willingness to accept low-ranking hosts. Genetic variation in host selection is also significant, though its impact on natural populations is less clear. Selective factors affecting host specialization are categorized into density-independent and density-dependent mechanisms. Density-independent factors include plant characteristics, natural enemies, and genetic trade-offs, which can favor either narrow or broad diet breadth. Density-dependent factors, such as competition for oviposition sites and enemy pressure, can also influence host specialization. The chapter concludes by highlighting the complex interplay of these factors in shaping the diet breadth of phytophagous insects.The chapter discusses the evolution and maintenance of host specialization in phytophagous insects, focusing on the factors that influence their diet breadth. Insects, being the most diverse group of organisms, have a wide range of host plants, and their host preferences are influenced by plant chemistry, ecological variables, and physiological states. The text outlines various models of host specialization, including optimality models, population genetic models, and physiological state models, which predict different behaviors and preferences based on factors such as egg load, search time, and plant quality. Proximate causes of intraspecific variation in host use are discussed, emphasizing the roles of learning, physiological state, and genetic variation. Learning can affect host acceptance, while physiological state influences the willingness to accept low-ranking hosts. Genetic variation in host selection is also significant, though its impact on natural populations is less clear. Selective factors affecting host specialization are categorized into density-independent and density-dependent mechanisms. Density-independent factors include plant characteristics, natural enemies, and genetic trade-offs, which can favor either narrow or broad diet breadth. Density-dependent factors, such as competition for oviposition sites and enemy pressure, can also influence host specialization. The chapter concludes by highlighting the complex interplay of these factors in shaping the diet breadth of phytophagous insects.