"It is said that earliest life originated in water but now we are facing danger of being extinct due to contaminated water".So this book is focused on Microcystis aeruginosa and their toxic effect on fish life.Microcystis is a unicellular, colonial cyanobacterium commonly found in freshwater habitats. This algae characteristically form dense unialgal growth often referred to as a bloom or "tides". Serious illness such as hepatoenteritis, a symptomatic pneumonia and dermatitis may result from consumption or contact with water contaminated with toxin producing cyanobacteria. Microcystis produces toxin known as "microcystin" and this toxin is responsible for human illness, bird mortality and extensive fish kills. Generally microcystin is hepatotoxic in nature. The entry of microcystin into hepatocytes of liver and other targeted tissues is accomplished by the broad specificity anion transport bile acid carriers. Extensive research has shown the impact of Microcystis extract resulted apoptosis, necrosis, pycnotic nuclei, blood depletion, hemorrhage, swelling in fish liver, gills, kidney, gonads and showed variations in biochemical, haematological and blood protein studies.