In the study of transmembrane transport, molecular phylogeny provides a reliable guide to protein structure, catalytic and noncatalytic transport mechanisms, mode of energy coupling and substrate specificity. It also allows prediction of the evolutionary history of a transporter family, leading to estimations of its age, source, and route of appearance. Phylogenetic analyses, therefore, provide a rational basis for the characterization and classification of transporters. A universal classification system has been described, based on both function and phylogeny, which has been designed to be applicable to all currently recognized and yet-to-be discovered transport proteins found in living organisms on Earth.