MAPK cascades are highly conserved signaling modules in eukaryotes. Arabidopsis MPK3 and MPK6, two closely related MAPKs, are involved in signaling multiple defense responses and play essential roles in plant growth and development. Our research is focused on the molecular mechanisms of MPK3/MPK6 cascade in signaling plant growth, development, and defense response. By understanding, at molecular and cellular levels, how MAPK cascades function as molecular switches in response to receptors/sensors and transduce signal from extracellular stimuli to downstream responses, we could eventually identify suitable targets for genetic engineering of crops with enhanced yield or disease resistance.