Cyanobacteria, formerly called ”blue-green algae“, are simple, primitive photosynthetic microorganism wide occurrence in fresh, brackish and salt waters. Forty different genera ofCyanobacteriaare known and many of them are producers of potent toxins responsible for a wide array of human illnesses, aquatic mammal and bird morbidity and mortality, and extensive fish kills. These cyanotoxins act as neurotoxins or hepatotoxins and are structurally and functionally diverse, and many are derived from unique biosynthetic pathways. All known cyanotoxins and their chemical and toxicological characteristics are presented in this article.