Supplementation of diets for weanling pigs with organic acids (e.g. citric, formic, fumaric and propionic acid) has been shown, in many cases, to increase average daily weight gain (ADG, range 4 to 27%) and gain to feed ratio (G/F, range 3 to 15%) and reduce the incidence of diarrhea. However, organic acid supplementation may not always improve ADG or G/F. Potential reasons for inconsistent results are briefly discussed. The growth promoting effects of organic acids may be attributed to increased nutrient and energy retention and digestibility (up to 4%), alteration of bacterial populations and metabolites in the gastrointestinal tract and(or) an effect on metabolism. More research on the physiological, microbial and biochemical effects of supplementing diets for weanling pigs with organic acids is necessary to provide better understanding of their mode of action.