Publisher Summary The chapter focuses on the production of lactic acid by microbiological processes. It discusses the chemical and physical properties of the acid followed by its commercial uses. Lactic acid, 2-hydroxypropanoic acid or α-hydroxypropanoic acid, is served as a food preservative in fermented milks, fruits, vegetables, meats, and breads. Various carbohydrate raw materials are used for lactic acid production by bacteria and molds such as refined carbohydrate raw materials; glucose, sucrose, and starch; complex carbohydrates—cellulose, cereal grains, corn, Jerusalem artichokes, and potatoes; and waste materials—cheese whey and permeate, municipal solid waste, and wood molasses. Factors affecting lactic acid concentrations, productivities, and yields include types of process (batch, fed-batch, or continuous), microorganism, strain, inoculum size and nutritional state, temperature, pH, substrate concentration and pretreatment, the presence of competing contaminants in non sterile systems, the presence of bacteriophages, and inhibition by lactic acid with increasing concentrations and by toxic products from substrate pretreatment such as furfural and hydroxymethylfurfural. The chapter also discusses various methods of product recovery and purification namely precipitation and acidification, centrifugation and microfiltration/ultrfiltration, distillation, ion exchange and adsorption, reverse osmosis, and solvent extraction and extractive fermentation. Various types of process controls are also summarized.