The T cell repertoire is shaped by positive and negative selection of thymocytes that express low levels of T cell receptor (TCR) and both CD4 and CD8. TCR-mediated signals that determine these selection processes are only partly understood. Vav, a GDP-GTP exchange factor for Rho-family proteins, is tyrosine phosphorylated following TCR stimulation, suggesting that it may transduce TCR signals. We now demonstrate that mice lacking Vav are viable and display a profound defect in the positive selection of both class I- and class II-restricted T cells. In contrast, Vav is not essential for negative selection, though in its absence negative selection is much less effective. Vav may influence the efficiency of TCR-induced selection events by regulating the intracellular calcium flux of thymocytes.