Fungal diseases in fish pose great risks to aquaculture and wild fisheries, which usually have severe economic and ecological consequences. This chapter explores the prevalence, causes, and diagnosis of fungal infections in fish, thus providing a comprehensive understanding of their effects on aquatic systems. Fungal diseases are usually opportunistic pathogens such as Saprolegnia and Aphanomyces, which strike at weakened or stressed fish. Environmental factors such as poor quality of water, overcrowding, and injuries contribute to an increased susceptibility to such infections. The chapter outlines the clinical signs and pathological features of fungal diseases, including skin erosions, fin erosion, and systemic infections, which are some of the highly debilitating health issues for fish. The diagnostic methods explained in detail are the macroscopic examination up to elaborate techniques such as microscopy, culture techniques, and molecular diagnostics with a focus on polymerase chain reaction (PCR). All these methods allow the correct identification of fungal pathogens and distinguish them from other microbial diseases. Sampling protocols and laboratory handling techniques will be discussed to ensure integrity at the specimen level. Strategies like improving the quality of water, removal of stresses, and antifungal treatments have been featured for effective management of outbreaks. In this chapter, the knowledge of fungal pathogenesis, environmental management, and diagnostic studies has been integrated to equip aquaculture practitioners and researchers with the tools to identify and control fungal diseases. It thus highlights a proactive approach for controlling diseases in fish populations to improve fish welfare and sustainability in aquaculture.