This chapter analyzes the mechanism of nutrient fluxes and regulation in the fish intestine. The fish intestine shares many fundamental nutrient transport properties with the mammalian intestine, but it cannot be tacitly assumed that this is uniformly the case. Particularly for amino acid (AA) transport, there are many unsolved questions about the number and specificity of the pathways involved. The chapter examines the cellular mechanisms responsible for nutrient absorption as a logical background for discussing adaptive changes in transport mechanisms. It also explores the recent advances made in the molecular biology of glucose transport and its relevance to the fish intestine. Work in this area for AA transport has recently emerged and no information is yet available in fish species. Despite these advances in mammalian species, the chapter focuses on the question that whether the knowledge of transport mechanisms is sufficient in fish to pinpoint the mechanisms that underlie adaptive changes in nutrient absorption. It also discusses the adaptive regulation of nutrient absorption, emphasizing two major aspects: adaptations to diet and hormonal regulation.