The proton-linked monocarboxylate transporter (MCT) family plays a crucial role in cellular metabolism and inter-tissue communication by facilitating the transport of monocarboxylates such as lactate and pyruvate across the plasma membrane. Nine MCT-related sequences have been identified in mammals, with varying tissue distributions. Six related proteins are found in Caenorhabditis elegans, and four in Saccharomyces cerevisiae. MCT1 is ubiquitously expressed, especially in heart and red muscle, where it is up-regulated during increased work, suggesting a role in lactic acid oxidation. MCT4 is prominent in white muscle and other high glycolytic cells, such as tumor cells and white blood cells, indicating its role in lactic acid efflux. MCT2 has a higher affinity for substrates and is found in cells requiring rapid uptake at low concentrations, including the proximal kidney tubules, neurons, and sperm tails. MCT3 is uniquely expressed in the retinal pigment epithelium. Regulation of MCT isoforms involves alternative splicing and promoter usage, with MCT1 and MCT4 interacting with OX-47 (CD147). The MCT family is essential for metabolic communication and has wide-ranging implications for health and disease. The review discusses the identification, cloning, and sequencing of MCT1, MCT2, MCT3, MCT4, and other isoforms, highlighting their tissue distributions, functional roles, and regulatory mechanisms. The MCT family includes nine members in mammals, six in C. elegans, and four in S. cerevisiae. The evolutionary relationships among MCT isoforms are discussed, with MCT3 in chickens being more closely related to mammalian MCT4 than MCT3. The structure and function of MCT isoforms, including their topology, glycosylation, and substrate specificity, are analyzed. The review also addresses the physiological roles of different MCT isoforms, their tissue distributions, and their involvement in metabolic processes. The MCT family is essential for the transport of monocarboxylates and has significant implications for cellular metabolism and disease.The proton-linked monocarboxylate transporter (MCT) family plays a crucial role in cellular metabolism and inter-tissue communication by facilitating the transport of monocarboxylates such as lactate and pyruvate across the plasma membrane. Nine MCT-related sequences have been identified in mammals, with varying tissue distributions. Six related proteins are found in Caenorhabditis elegans, and four in Saccharomyces cerevisiae. MCT1 is ubiquitously expressed, especially in heart and red muscle, where it is up-regulated during increased work, suggesting a role in lactic acid oxidation. MCT4 is prominent in white muscle and other high glycolytic cells, such as tumor cells and white blood cells, indicating its role in lactic acid efflux. MCT2 has a higher affinity for substrates and is found in cells requiring rapid uptake at low concentrations, including the proximal kidney tubules, neurons, and sperm tails. MCT3 is uniquely expressed in the retinal pigment epithelium. Regulation of MCT isoforms involves alternative splicing and promoter usage, with MCT1 and MCT4 interacting with OX-47 (CD147). The MCT family is essential for metabolic communication and has wide-ranging implications for health and disease. The review discusses the identification, cloning, and sequencing of MCT1, MCT2, MCT3, MCT4, and other isoforms, highlighting their tissue distributions, functional roles, and regulatory mechanisms. The MCT family includes nine members in mammals, six in C. elegans, and four in S. cerevisiae. The evolutionary relationships among MCT isoforms are discussed, with MCT3 in chickens being more closely related to mammalian MCT4 than MCT3. The structure and function of MCT isoforms, including their topology, glycosylation, and substrate specificity, are analyzed. The review also addresses the physiological roles of different MCT isoforms, their tissue distributions, and their involvement in metabolic processes. The MCT family is essential for the transport of monocarboxylates and has significant implications for cellular metabolism and disease.