In the 1950s and 1960s, concern grew about the ‘food gap’ between the industrialized and the less industrialized parts of the world, especially considering the rapid and continuing population growth in the latter. The hope was that protein from microorganisms (single cell protein, SCP) would help meet this world protein deficiency. SCP has a number of attractive features. Microbes are not subject to the vicissitudes of weather, they have a much more rapid growth and can be grown using waste products. One concern is that SCP microorganisms contain high levels of RNA and their consumption could lead to uric acid accumulation. However, microorganisms have been consumed in large amounts, either wholly or as part of a meal or alcoholic beverage, for centuries. Fermented milks and yoghurts which have been consumed from ancient times up to the present day contain large amounts of bacteria and yeasts. This chapter discusses single cell protein production, substrates such as hydrocarbons, alcohols, waste products, use of autotrophic microorganisms, safety of single cell protein, nucleic acids and their removal from SCP, and nutritional value of single cell protein.