Mitogen-activated protein kinase (MAPK) cascades are key signaling modules downstream of receptors/sensors that perceive either endogenously produced stimuli such as peptide ligands and damage-associated molecular patterns (DAMPs) or exogenously originated stimuli such as pathogen/microbe-associated molecular patterns (P/MAMPs), pathogen-derived effectors, and environmental factors. In this review, we provide a historic view of plant MAPK research and summarize recent advances in the establishment of MAPK cascades as essential components in plant immunity, response to environmental stresses, and normal growth and development. Each tier of the MAPK cascades is encoded by a small gene family, and multi ple members can function redundantly in an MAPK cascade. Yet, they carry out a diverse array of biological functions in plants. How the signaling specificity is achieved has become an interesting topic of MAPK research. Future investigations into the molecular mechanism(s) underlying the regulation of MAPK activation including the activation kinetics and magnitude in response to a stimulus, the spatiotemporal expression patterns of all the components in the signaling pathway, and functional characterization of novel MAPK substrates are central to our understanding of MAPK functions and signaling specificity in plants.