Studies of the early events that follow pathogen recognition have established the importance of mitogen-activated protein kinase (MAPK) cascades in plant defense signaling. Specific MAPK cascade components have now been identified in several species, and so it has become possible to compare defense responses to diverse pathogens and to elucidate downstream events that are initiated by these key signaling molecules. Recent work utilizing post-transcriptional gene silencing techniques and the development of stable transgenic lines for both gain- and loss-of-function analysis have further advanced our understanding of the role of MAPK cascades in plant immunity.