TCR gamma delta + T cells represent a minority of CD3+ T cells in many species including man. The molecular structure of the TCR gamma and delta loci in man is well understood. The gamma and delta loci contain V, D, J and C gene segments. These segments do not rearrange randomly but in a coordinated, ordered fashion during thymic development. Therefore, the structure of gamma and delta genes of early fetal TCR gamma delta + thymocytes differ drastically from those in postnatal TCR gamma delta + thymocytes. In contrast to postnatal TCR gamma delta + thymocytes, early fetal-TCR gamma delta + produce substantial levels of IL-4 and IL-5 and the possibility is discussed that the early fetal TCR gamma delta + cells are involved in development of TCR gamma delta + cells. In man, unlike in mouse, no preferential homing of early fetal TCR gamma delta + cells has been observed so far. Mature human peripheral TCR gamma delta + cells can recognize a great variety of cell surface antigens including 'classical' and 'non-classical' MHC antigens, immunoglobulins and other undefined antigens. In addition, TCR gamma delta + can recognize bacterial products. So far, no class of antigens has been defined that is preferentially recognized by TCR gamma delta + T cells and the function of these cells remains elusive.