This paper discusses in general terms the mechanisms proposed to explain the resistance of higher plants to infection and colonization by bacteria and fungi, especially the type of resistance associated with rapid killing of host cells and very limited growth of the pathogen; that is, with the hypersensitive reaction of the plant to infection (h.r.). The main points considered are: the significance of the h.r. for obligate and other types of parasites; the nature of the h.r. and the means by which it is invoked; the absence of the h.r. in compatible host-parasite combinations; the h.r. and the production of phytoalexins, and the role of the latter in resistance; the part played by cell-wall degrading enzymes in the earliest stages of infection; the relation between the h.r. in interactions between races of a pathogen and different forms of a host species, on the one hand, and the resistance of all plants to almost all pathogens, on the other. Also discussed is the less specific but important type of resistance in which the pathogen continues to grow in resistant plants more slowly than in susceptible plants.