The selection of functional T cells is mediated by interactions between the T cell antigen receptor and self-peptide major histocompatibility complex expressed on thymic epithelium. These interactions either lead to survival and development or death. The T cell antigen receptor is an unusual receptor able to signal multiple cell fates. The precise mechanism by which this is achieved has been an area of intense research effort over the years. One model proposes that the differential activation of mitogen-activated protein kinase pathways contributes to this decision. Here, the role of extracellular signal-regulated kinase in promoting or preventing apoptosis during thymic selection is discussed.