Organic acids and their salts appear to be potential alternatives to prophylactic in-feed antibiotics and growth promoters in order to improve the performance of weaned piglets, fattening pigs and reproductive sows, although their growth-promoting effects are generally less than that of antibiotics. Based on an analysis of published data, the growth-promoting effect of formates, fumarates and citrates did not differ in weaned piglets. In fattening pigs, formates were the most effective followed by fumarates, whereas propionates did not improve growth performance. These acids improved the feed : gain ratio of both weaned piglets and fattening pigs. In weaned piglets, the growth-promoting effects of dietary organic acids appear to depend greatly on their influence on feed intake. In sows, organic acids may have anti-agalactia properties. Successful application of organic acids in the diets for pigs requires an understanding of their modes of action. It is generally considered that dietary organic acids or their salts lower gastric pH, resulting in increased activity of proteolytic enzymes and gastric retention time, and thus improved protein digestion. Reduced gastric pH and increased retention time have been difficult to demonstrate, whereas improved apparent ileal digestibilities of protein and amino acids have been observed with growing pigs, but not in weaned piglets. Organic acids may influence mucosal morphology, as well as stimulate pancreatic secretions, and they also serve as substrates in intermediary metabolism. These may further contribute to improved digestion, absorption and retention of many dietary nutrients. Organic acid supplementation reduces dietary buffering capacity, which is expected to slow down the proliferation and/or colonization of undesirable microbes, e.g. Escherichia coli, in the gastro-ileal region. However, reduced scouring has been observed in only a few studies. As performance responses to dietary organic acids in pigs often varies, more specific studies are necessary to elucidate an explanation.