Oomycete plant pathogens, such as Phytophthora, downy mildews and Pythium, have devastating disease effects on numerous crop and ornamental plants. Various types of genetic resistance to oomycetes occur in plants, and can be determined at the subspecific or varietal level (race or cultivar-specific resistance), or at the species or genus level (nonhost resistance). In addition, resistance might be a quantitative phenotype (partial resistance). Resistance reactions are often associated with the hypersensitive response - a programed cell death pathway. Recent advances in the genetic, biochemical and cytological characterization of disease resistance suggests that the hypersensitive response is associated with all forms of resistance to Phytophthora and downy mildews. Identification of the resistance genes involved in nonhost and partial resistance to oomycetes remains an important challenge.