This review discusses in considerable detail the tyrosine phosphorylation events that couple the T cell antigen receptor to downstream signaling pathways. First, the protein kinases that catalyze these tyrosine phosphorylation are introduced, then the phosphatases that mediate removal of phosphate from tyrosine residues. Next, we discuss the molecular clock work by which these enzymes initiated T cell activation. Finally, we briefly review the key substrates for tyrosine phosphorylation and their role in coupling the kinases and phosphatases to gene transcription and other aspects of the T lymphocyte response to antigen.