Rust fungi are obligately biotrophic plant parasites that obtain their nutrients from living host cells. The initiation of the two parasitic phases of these fungi generally requires topographic signals from the plant surface followed, for the dikaryotic phase, by a successive sequence of signals to control further fungal development within the plant. During the fungal life cycle, three types of intracellular structures (invasion hyphae, M-, and D-haustoria) are formed and each may differently affect the host membrane that surrounds it, as well as affecting other cellular components. Each intracellular structure also prevents non-specific plant defences triggered by fungal activities, possibly by interfering with the signalling system rather than defence expression. In resistant host cultivars, cellular invasion triggers a rapid cell death (the hypersensitive response) that shares some features with developmentally programmed cell death in animal and plant tissues, and is controlled by parasite-specific resistance genes that resemble those that defend plants against other types of pathogens. Evidence from one system suggests that this response is specifically elicited by a fungal peptide and does not involve the oxidative burst typical of resistance expression in other plant-pathogen interactions. However, overall, few of the molecules involved in any of these plant-rust fungi interactions have been completely characterized and much is left to be discovered, particularly with respect to how cellular susceptibility to rust fungi is conditioned. Copyright 1997 Annals of Botany Company