This chapter provides an insight into the great diversity of behavioural defenses in fish and the mechanisms shaping this diversity. It focuses on the diverse anti-predator tactics exhibited by fishes. The chapter presents understanding of behavioural defense strategies in fishes. All behavioural actions are the end results of complex interactions between genetic and environmental factors. Fish are often remarkably plastic in their behaviour, altering strategies in response to a changing environment where learning ability often plays a crucial role in the development of anti-predator behaviour. Artificial selection can also alter anti-predator behaviour as shown in experiments comparing wild-type and domesticated salmonids. Although the knowledge about flight behaviour in fish is increasing, information is still scarce about the fitness consequences of different flight tactics. Behavioural avoidance is a first line of defense against parasites, which if successfully employed will reduce the demands on the immune system.