The class Oomycetes (which means egg fungi) is a group of organisms primarily microscopic with a worldwide distribution, inhabiting terrestrial, freshwater, and to a lesser extent, oceanic environments. They are organisms that often are unnoticed but they are ecologically important, either because they parasitize animals and plants or because they feed on organic debris. Many species cause serious diseases in animals and plants, including commercially important species.

They are small organisms, generally microscopic, often formed by more or less branched filaments of single-cell thickness, with few or no septa (transverse cell walls). These filaments produce substances that dissolve food, allowing it to enter to the host cell. Their life cycles are dominated by the diploid phase, which has two copies of each chromosome. They frequently form asexual spores, enabling them to reproduce massively under suitable conditions. Additionally, they can form sporangia that produce swimming cells of asexual origin, which can move, attach to a substrate, and grow. Sexually mature individuals typically form female structures consisting mainly of large cells called oogonia, each with one copy of each chromosome. These cells are fertilized by the tips of filaments from another individual or the same individual, which act as sperm cells (called antheridium) as they possess only one copy of each chromosome. When the antheridium touches the oogonium, the cell walls fuse, and the nuclei from the antheridium pass into the oogonium, merging and forming cells with two copies of each chromosome. The oogonium eventually forms spores that can germinate rapidly or remain as resistance structures.