Mitogen-activated protein kinases (MAPKs) are activated by a wide variety of cellular stimuli and involved in the regulation of most, if not all, cellular processes. Among them, the c-Jun N-terminal kinase and p38 MAPKs are predominantly induced in response to proinflammatory cytokines and most environmental stresses, such as pathogenic insults, heat and osmotic shock, ultraviolet light, protein synthesis inhibitors, DNA-damaging agents, and withdrawal of growth factors; thus, they are also known as stress-activated protein kinases. In this chapter we describe alternative methods designed to measure the activity of these stress-activated protein kinases, c-Jun N-terminal kinase, and p38 MAPKs.