Fungal pathogens have evolved elaborate strategies to gain access to plant tissues. For many pathogens, following attachment of spores to the leaf surface, germ tubes emerge and grow across the surface, often in response to particular environmental cues and to a specific location. At an appropriate site, polar elongation of the germ tube ceases, the tip attaches to the surface and swells to form an appressorium, a uniquely organized infection structure. Following a period of maturation, a hypha then emerges at the plant interface and penetrates into the plant tissues. This chapter discusses recent developments that provide new insight into the molecular mechanism regulating induction and function of appressoria. Topics include attachment to the leaf surface; environmental cues that signal germ-tube growth and appressorium formation; mechanisms for sensing environmental cues; endogenous signaling pathways; and mechanisms of penetration from the appressorium.