Phytophthora diseases cause widespread economic and environmental losses worldwide. Thousands of plant species are susceptible. Disease is typically initiated through the activity of motile, biflagellate zoospores. Plant penetration and colonisation are achieved through the secretion of a diverse range of cell wall-degrading enzymes and effector proteins. Effector proteins are especially important during biotrophic growth; they function to suppress host defence and regulate host metabolism to favour pathogen growth. Plants can detect the presence of Phytophthora cells and rapidly mount a basal defence response that often successfully inhibits disease development. A key aspect of basal defence is the formation of wall appositions that constitute a physical and chemical barrier to pathogen growth. Components of basal defence can be triggered by chemical and physical signals produced by invading Phytophthora cells. If basal defence fails to inhibit pathogen ingress, the plant cell under attack can undergo hypersensitive cell death. In Phytophthora–plant interactions, hypersensitive cell death can be triggered by elicitors or effectors. In the continuing arms race between pathogen and plant host, Phytophthora species have evolved a range of counter-defence mechanisms that include suppression of hypersensitive cell death, inhibition of plant degradative enzymes and protection against reactive oxygen species. This brief article provides an overview of the contribution of modern molecular cytology to our understanding of Phytophthora–plant interactions.