Aquaculture is the world's fastest developing food production sector. The industry has experienced rapid growth in recent years as global catches from wild fisheries have reached maximum sustainable yields or declined. Lactococcosis is one of the most serious emerging diseases in fish worldwide. Outbreaks are associated with high mortality rates and significant economic losses, and effective tools for treatment and prevention are limited. A wide range of host species are susceptible, including wild and cultured fish from cold, temperate, and warm fresh or marine water systems. Infections have also been reported in humans and other terrestrial species, making it a disease of concern in aquaculture, conservation, and human and animal health. This lecture will summarize the research efforts by the UC Davis Aquatic Animal Health laboratory to better understand the epizootiology of piscine lactococcosis and develop safe and effective prophylactic and therapeutic approaches against this disease.