World aquaculture is the fastest growing animal food producing sector with an annual growth rate of 6.6%. However, the high density stocking densities and large scale commercial farming system offer considerable challenges for health control. Bacterial, viral and parasitic infections may cause significant losses in aquacultural enterprises but vaccination methods implemented since the 1990s have demonstrated their role as one of the most efficient disease control strategies. These have been particularly successful with regard to bacterial diseases in Norwegian salmon farming where multivalent vaccines have reduced the need for usage of antibiotics with more than 99 % since the 1980s. Fish can be vaccinated by three different administration routes: injection, immersion and oral vaccination. Injection vaccination (intraperitoneal injection of vaccine) is the most time consuming and labor intensive method, which however, provides the best protection of the fish. Immersion vaccination is used for immunization of a high number of small fish is cost-efficient and fast (30 sec immersion into vaccine). Oral vaccination (vaccine in feed) is the least efficient. As in higher vertebrates fish respond to vaccination by increasing the specific antibody titer and by activating the cellular responses. My talk will cover vaccination methods in fish, immune responses and some adverse effect of oil-adjuvanted vaccines in fish with reference to our work in rainbow trout, Oncorhynchus mykiss.