Heat shock of mammalian cells causes protein damage and activates a number of signaling pathways. Some of these pathways enhance the ability of cells to survive heat shock, e.g., induction of molecular chaperones [heat shock protein (HSP) HSP72 and HSP27], activation of the protein kinases extracellular signal-regulated kinase and Akt, and phosphorylation of HSP27. On the other hand, heat shock can activate a stress kinase, c-Jun NH2-terminal kinase, thus triggering both apoptotic and nonapoptotic cell death programs. Recent data indicate that kinases activated by heat shock can regulate synthesis and functioning of the molecular chaperones, and these chaperones modulate activity of the cell death and survival pathways. Therefore, the overall balance of the pathways and their interplay determine whether a cell exposed to heat shock will die or survive and become stress tolerant.