Microalgae are microscopic, mostly single-celled algae such as Chlorella, Dunaliella, Haematococcus, and the cyanobacterium Spirulina. They are among the fastest-growing sources of protein, omega-3 fatty acids, carotenoids, and natural pigments in the food world, with protein contents that can exceed fifty percent of dry weight. Approved species are sold as supplements and food ingredients, their oils enrich infant formula and functional foods, and pigments like astaxanthin color aquaculture feeds. Safety assessment covers heavy metals, microcystin toxins in wild harvests, and allergenicity before novel microalgal ingredients reach the market, and cultivation in closed photobioreactors gives the cleanest, most consistent product.