This paper reviews the biochemical background of metabolic acidoses. The rate of development is judged from production and/or elimination rate of organic acids, particularly carboxylic acids, namely lactate, ketoacids, acetate, formate and glycollate. Further, acid production from changes in the chemical state of phosphate in tissue is evaluated. The main conclusion is that pathological conditions with acidoses are always accompanied by changes in the rate of elimination of the carboxylic acids, whereas changes in the chemical state of phosphate is of quantitatively minor importance. Further, metabolic effects of metabolic acidoses are described with special reference to the effect of low pH in the extra cellular fluid on glycolysis, gluconeogenesis, lipolysis and ketogenesis. A short outline of the differential diagnostic problems in metabolic acidoses due to changes in carbohydrate and lipid metabolism or intoxication is given.