Mitogen-activated protein (MAP) kinase cascades are conserved among all eukaryotes. Plant genomes encode many components of the MAP kinase cascade, suggesting divergent roles in mediating various external and internal signals. In this chapter, we summarize the roles for MAP kinase cascades involved in plant cell division and development. Recently, a MAP kinase pathway, called the NACK-PQR pathway, has been shown to directly control the expansion of the phragmoplast by phosphorylating MAP65-1. In addition, there has been an advance in the understanding of the roles of MAP kinase pathways in embryogenesis, development of stomata and root hairs, and transmission of signals generated by ethylene and auxin. © 2008 Springer-Verlag Berlin Heidelberg.