This chapter reviews associations between physiology and several classifications of fish behaviour. Learning is an important component of behavioural plasticity during foraging in fish, and fish that can learn the location or timing of food sources while minimizing the risk of predation and competition should have an advantage. Temperature has a profound effect on fish foraging behaviour, generally increasing feeding rates with warming until the fish approaches its upper lethal limits, where activity is reduced. The presence of a predatory threat can have various effects on fish physiology. In response to hypoxia, fish can either decrease activity, to reduce energy expenditure, or increase spontaneous swimming as they attempt to relocate to an area with increased oxygen availability. The relationships between physiology and behaviour in fishes consist of a complex array of control systems and feedbacks that determine the phenotype that is exposed to the environment and ultimately, selection.