Genetic modification of microorganisms has been applied to the production of food enzymes such as chymosin, and to the optimization of traditional microorganisms used in the fermentation of food such as baker's yeast and lactic acid bacteria. Chymosin produced with genetically modifiedEscherichia coli K12, Kluyveromyces lactis, andAspergillus nigersubsp.awamorihas been accepted by some markets in about 20 countries and therefore entered the human food chain. However, genetically modified baker's yeast has it much harder achieving acceptance although it has been approved in the United Kingdom. Genetic engineering of food systems is not a matter of the inventing scientists and industries only, it is as much a matter of acceptance by the consumer and the general public. It is the scope of this review to describe the biotechnology of the chymosin and baker's yeast systems, and to outline some other systems such as the lactic acid bacteria. The technical description is supplemented with information on safety assessment, regulation, and labeling, as well as the theory and practice of risk perception and acceptance of such products by the general public.