Mitogen-activated protein kinase (MAPK) pathways are protein kinase cascades that have a function in the transduction of extracellular signals to intracellular targets in all eukaryotes. Distinct MAPK pathways are regulated by different signals and have a role in a wide variety of physiological processes. In plants there is evidence for a role of MAPKs in the signaling of pathogens, abiotic stresses, plant hormones, and cell cycle cues. A large number of distinct MAPKs in plants have been identified that are all most similar to the animal ERK MAPKs. By sequence alignment all available full length plant MAPKs can be grouped into five subfamilies. Functional data exist for members of four subfamilies and show that different subfamilies encode MAPKs for specific functions. Analysis of partial MAPK sequences from full length, truncated cDNAs and expressed sequence tags (ESTs) revealed the presence of two new subfamilies in the plant MAPK superfamily. Signature sequences valid for the superfamily of plant MAPKs and each subfamily were derived and should help in future classification of novel MAPKs. The future challenge is to unambiguously assign functions to each MAPK and decipher the other partners of their signaling pathways.