Assembly, expression, and signal transduction by the pre-T-cell receptor (pre-TCR) and TCR complexes are critical for normal thymocyte development. How environmental cues sensed by these two receptor complexes are translated into biological responses that result in the generation of functionally mature T cells is increasingly better understood. Invariant CD3gamma, delta, epsilon, and zeta polypeptides are central to the function of these two receptor complexes. CD3 chains ensure correct intracellular assembly, surface expression, and signal transduction via the receptors in ligand-independent and -dependent manners. In the present review, the roles of the CD3 chains, particularly CD3gamma, delta, and epsilon, in thymocyte differentiation in mice and humans are reviewed.