Mitogen-activated protein kinases (MAPKs) link information transfer from external stimuli-activated sensors to cellular responses. The completed Arabidopsis genome sequence revealed an extraordinary complexity in MAPK-signalling components in plants. Information obtained from Arabidopsis provides a framework for a unified nomenclature and the assembly and function of MAPK-signalling pathways. Strategies and tools are evolving to connect MAPK pathways and to determine their function. As a result, MAPK signalling modules emerged, one of which appears to antagonistically regulate stress- and growth-responses and another that regulates cytokinesis.