Mitogen-activated protein (MAP) kinases, which include the extracellular response kinases, p38 and c-Jun amino terminal kinases (JNK), play a significant role in mediating signals triggered by cytokines, growth factors and environmental stress. The JNK and p38 MAP kinases have been involved in growth, differentiation and cell death in different cell types. In the present paper, we describe how the JNK and p38 MAP kinase signalling pathways are regulated and their role during thymocyte development and the activation and differentiation of T cells in the peripheral immune system. The results from these studies demonstrate that the JNK and p38 MAP kinase signalling pathways regulate different aspects of T-cell mediated immune responses.