Oomycetes are a diverse group of eukaryotic organisms that have colonised many ecological niches; yet more than 60% of the known species are parasitic on plants. Parasitism of plants has evolved several times independently in three different lineages of the Oomycota. Here, we provide an overview of the current knowledge of the diversity, evolution and lifestyles of plant parasitic oomycetes. We then report on recent advances in molecular studies on oomycete–plant interactions with a particular emphasis on work with oomycete effectors. In the future, genome sequencing of a broader spectrum of oomycete species will expand our knowledge of pathogenicity mechanisms and will likely reveal novel structural and functional classes of effectors.