Many fungal pathogens of plants develop highly specialized infection structures in response to signals detected upon contact with the plant surface. In addition, proliferation within host tissues requires both metabolic and morphological adaptation to the plant environment. Recent work on several different phytopathogenic fungi has revealed a role for cAMP-mediated signal transduction in these processes. The general finding is that mutants with defects in various components of the cAMP signaling pathway have attenuated virulence. These results expand the list of fungal virulence factors to include signaling components required for an appropriate interaction with the host.