The T cell antigen receptor (TCR) transduces signals that mediate different responses depending on the stage of development of the T cell and the nature of the ligand it engages. The presence of multiple signal transducing subunits (CD3-γ-δ,-εandζchain) suggests the potential to control these responses by altering the subunit composition of the TCR.ζchain represents an especially important signalling molecule as it contains multiple signalling motifs within its cytoplasmic tail. The generation and analysis ofζdeficient (ζ−/−) andζ-transgenic mice has provided insight into the role ofζas well as the CD3 subunits in TCR surface expression, T cell activation and thymocyte development. Herein, we discuss the results from such experiments which suggest distinct roles forζchain and the CD3 components at different stages of T cell development.