hot springs and ice-cold water. Most cyanobacteria are an immense source of several secondary natural products with applications in the food, pharmaceuticals, cosmetics, agriculture, and energy sectors. Moreover, some species of cyanobacteria grow vigorously and form a dominant microflora in terms of their biomass and productivity in specific ecosystems. Bloom formations due to excessive growth of certain cyanobacteria followed by the production of toxic compounds have been reported in many eutrophic to hypertrophic lakes, ponds, and rivers throughout the world. A range of toxic secondary compounds, called cyanotoxins, have been reported from cyanobacteria inhabiting freshwater and marine ecosystems. These toxic compounds are highly detrimental for survival of several aquatic organisms, wild and/or domestic animals, and humans. Aquatic organisms, including plants and animals, as well as phytoplankton and zooplankton inhabiting under toxic bloom rich ecosystems, are directly exposed to the harmful effects of different cyanotoxins. The intoxication occurring in wild and/or domestic animals and humans is either due to direct ingestion of cells of toxin producing cyanobacteria or the consumption of drinking water contaminated with cyanotoxins. The toxicity of different cyanotoxins is directly proportional to the growth of cyanobacteria and the extent of their toxin production. It has been shown that the growth of different cyanobacteria and their toxin biosynthesis is greatly influenced by different abiotic factors such as light intensity, temperature, short wavelength radiations, pH, and nutrients. Global warming and temperature gradients can significantly change species composition and favor blooms of toxic phytoplanktons. It has been assumed that cyanotoxins play an important role in chemical defense mechanisms giving survival advantages to the cyanobacteria over other microbes or deterring predation by higher trophic levels. Cyanotoxins may also take part in chemical signalling. Cyanotoxins are produced by cyanobacteria, a phylum of bacteria that obtain their energy through photosynthesis. The prefix cyan comes from the Greek κύανoς meaning "a dark blue substance", and usually indicates any of a number of colours in the blue/green range of the spectrum. Cyanobacteria are commonly referred to as blue-green algae. Traditionally they were thought of as a form of algae, and were introduced as such in older textbooks. However modern sources tend to regard this as outdated; they are now considered to be more closely related to bacteria, and the term for true algae is restricted to eukaryotic organisms. Like true algae, cyanobacteria are photosynthetic and contain photosynthetic pigments, which is why they are usually green or