Receptor-mediated signal transduction plays an important role in T-cell differentiation and function. The pre-T-cell receptor (pre-TCR) and TCR complexes are the most critical receptors for T-cell biology. Signals induced by pre-TCR and TCR in a ligand-independent or a ligand-dependent manner, respectively, are essential for thymocyte maturation. CD3 proteins, which are γ, δ, ε, and ζ polypeptides, play pivotal role in intracellular assembly, surface expression, and signal transduction via the pre-TCR and TCR complexes. Recent studies have suggested central and multiple roles for CD3ε in T-cell development and function. We review the role of the CD3ε chain in T-cell biology.