Highly proliferative cancer cells maintain high rates of glycolysis, producing large amounts of acids, mainly lactic acid. Despite this fact, only the interstitial pH of tumours is low, while the intracellular pH of tumours is either normal or higher than that of normal tissues. Monocarboxylate transporters (MCTs) are transmembrane proteins that, by promoting the efflux of the accumulating acids, constitute one of the most important mechanisms in the maintenance of tumour intracellular pH. MCTs play a central role in cellular metabolism and are essential for the transport of metabolically important monocarboxylates, such as lactate, across the plasma membrane.