γδ T cells remain an enigma. They are capable of generating more unique antigen receptors than αβ T cells and B cells combined, yet their repertoire of antigen receptors is dominated by specific subsets that recognize a limited number of antigens. A variety of sometimes conflicting effector functions have been ascribed to them, yet their biological function (s) remains unclear. On the basis of studies of γδ T cells in infectious and autoimmune diseases, we argue that γδ T cells perform different functions according to their tissue distribution, antigen-receptor structure and local microenvironment; we also discuss how and at what stage of the immune response they become activated.