Fisheries and aquaculture have increasingly contributed to nutrition and food security. However, disease outbreaks have significantly impacted fish production both in the wild and in aquaculture systems. The trend towards intensive aquaculture practices in confined spaces, such as tanks, ponds, and cages, aims to boost production but often results in stressful environments that negatively affect fish health and lead to infectious diseases. Despite efforts to use antibiotics and chemotherapeutics to control these diseases over the past two decades, these treatments pose sustainability and environmental challenges, including the development of antibiotic resistance in bacteria, which can harm aquaculture species. It is becoming evident that proper nutrition and immunostimulants are crucial for maintaining fish health and development under intensive conditions. Adequate levels of essential nutrients and immunostimulants are necessary for fish health. Various nutrients, bioactive additives, and feeding strategies can influence fish susceptibility to both infectious and non-infectious diseases. This chapter focuses on the need for vital nutrients, factors affecting fish health, nutritional disorders, and the role of immunostimulants in ensuring good fish health.