Technical Abstract: This is a book chapter on the genetics of Pythium to be published in a book titled "Oomycete Genetics and Genomics: Biology, Interactions with Plants and Animals, and Toolbox." It covers the basic taxonomic classification of species in the genus, the phylogenetic placements, and reviews the literature on genetic analysis, host-pathogen interaction, the ribosomal RNA genes, mitochondrial genomics and extrachromosomal elements.