Arthrospira is a photosynthetic, filamentous, spiral-shaped, multicellular and blue-green micro alga. Cell division occurs by binary fission. As it contains chlorophyll a, like higher plants, botanists classify it as a micro alga belonging to Cyanophyceae class; but according to bacteriologists it is a bacterium due to its prokaryotic structure. Mexicans (Aztecs) started using this microorganism as human food. Its chemical composition includes proteins (55%-70%), carbohydrates (15%-25%), essential fatty acids (18%) vitamins, minerals and pigments like carotenes, chlorophyll a and phycocyanin. Pigments are used in food and cosmetic industries. Spirulina is considered as an excellent food, lacking toxicity and have anticancer, antiviral, immunological properties and it also acts as a potent antioxidant. There has been a significant change in Spirulina functions under stress conditions.