Biotrophic fungal and oomycete pathogens have strikingly similar mechanisms to invade plants and retrieve nutrients. In particular, the obligate biotrophs form specialised feeding structures such as haustoria and intracellular hyphae that play an important role in host–pathogen signalling and nutrient retrieval. To establish a successful infection, pathogens need to overcome host defences, and suppression of plant defence responses is now a well-known phenomenon in disease. The molecular mechanisms of defence suppression by fungal and oomycete pathogens likely occurs via secreted or translocated effector proteins whose primary function is to establish compatibility. Many fungal and oomycete effector proteins are transported inside the host cell. In interactions with an obligate biotroph, the host provides the necessary nutrients for pathogen growth and reproduction. Specific transporters and metabolic enzymes in both host and pathogen contribute to nutrient exchange between the two organisms and hijacking of the host source–sink balance seems to be important for infection. Studies of plant–biotrophic fungal and plant–oomycete pathogen interactions, now exploiting a wealth of pathogen genome information, are providing unique insights into molecular mechanisms determining disease susceptibility and resistance.