Streptococcosis is an important disease of fish. It is commonly observed in freshwater fish although marine fish have also been reported to suffer from the disease. It is caused by bacterial species from the genus Streptococcus and has been reported in many countries, especially in the regions that extensively practicing aquaculture as the main protein source, such as Asia, Middle East and South American. Many species of Streptococcus have been associated with this disease, but Streptococcus agalactiae and S. iniae are the most frequently isolated species from diseased fishes. This book compiles, describes, and analyses the current literatures on streptococcosis in fish throughout the world. It involves brief description of the aquaculture industry, selected Streptococcus spp. as the causative organism, the prevalence, risk factors and transmission of streptococcosis in fish. The clinico-pathological changes and diagnostic techniques for the disease, the economic impacts and control protocols of streptococcosis in the aquaculture industry are also discussed.