All metazoans possess innate immune defence system whereas parameters of the adaptive immune system make their first appearance in the gnathostomata, the jawed vertebrates. Fish are therefore the first animal phyla to possess both an innate and adaptive immune system making them very interesting as regards developmental studies of the immune system. The massive increase in aquaculture in recent decades has also put greater emphasis on studies of the fish immune system and defence against diseases commonly associated with intensive fish rearing. Some of the main components of the innate and adaptive immune system of fish are described. The innate parameters are at the forefront of immune defence in fish and are a crucial factor in disease resistance. The adaptive response of fish is commonly delayed but is essential for lasting immunity and a key factor in successful vaccination. Some of the inherent and external factors that can manipulate the immune system of fish are discussed, the main fish diseases are listed and the pathogenicity and host defence discussed. The main prophylactic measures are covered, including vaccination, probiotics and immunostimulation. A key element in the immunological control of fish diseases is the great variation in disease susceptibility and immune defence of different fish species, a reflection of the extended time the present day teleosts have been separated in evolution. Future research will probably make use of molecular and proteomic tools both to study important elements in immune defence and prophylactic measures and to assist with breeding programmes for disease resistance.