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Humans are continuously exposed to a high number of diverse pathogens that induce different types of immune responses. Primary pathogen-specific immune responses generate multiple subsets of memory T cells, which provide protection against secondary infections. In recent years, several novel T cell subsets have been identified and have significantly broadened our knowledge about T cell differentiation and the regulation of immune responses. At the same time the rapidly growing number of incompletely characterized T cell subsets has also generated some controversies. We therefore review here the current knowledge on features and functions of human α/β T cell subsets, focusing on CD4⁺ T cells classified according to cytokine production and tissue localization. The principal helper and regulatory T cell subsets can be identified by a limited number of relevant surface markers, which are an integral part …