Monocarboxylate transporters (MCTs) are members of the solute carrier (SLC) family (SLC16) of proteins. MCTs are present in a wide variety of tissues and are involved in the regulation of fundamental cellular processes, such as glycolysis, fatty acid homeostasis, as well as other key metabolic pathways. Of the 14 isoforms identified, proton-dependent MCTs 1–4 have been extensively studied due to their importance in transporting L-lactate, pyruvate, and short-chain fatty acids in a wide variety of tissues. Most importantly, many of these MCT isoforms are upregulated in tumor tissues, making them attractive targets and biomarkers for a wide variety of cancers.