The thesis describes the significance and impact of viral infections on cage fish farming. It also describes the six most common viral diseases that can significantly affect the health and productivity of fish in cage systems. Among the viral diseases, the following are described in detail: Infectious salmon anaemia, Viral haemorrhagic septicaemia, Encephalitis, Ulcerative dermal necrosis syndrome, Lymphocystis, and Viral nervous necrosis (VNN)/Viral encephalopathy and retinopathy (VER). For these diseases, the symptoms, pathogenesis and consequences for the fish populations are analysed. Prevention and control of viral diseases includes safety measures such as quarantine of new fish, disinfection of equipment and improved management practices. Vaccination, where available, also plays an important role in disease containment. This bachelor thesis emphasises the importance of monitoring fish health, ensuring preventative measures and developing new therapies and vaccines to reduce the impact of viral diseases on cage fish farming.