Water moulds or Oomycetes are a group of filamentous protists, physically resembling fungi. They are microscopic, absorptive organisms that reproduce both sexually and asexually and are composed of mycelia, or a tube-like vegetative body (all of an organism's mycelia are called its thallus). The name "water mold" refers to the fact that they thrive under conditions of high humidity and running surface water.

Water moulds were originally classified as fungi, but are now known to have developed separately and show a number of differences. Their cell walls are composed of cellulose rather than chitin and do not have septations. Also, in the vegetative state they have diploid nuclei, whereas fungi have haploid nuclei.

Instead, water moulds are related to organisms such as brown algae and diatoms, making up a group called the heterokonts. The name comes from the common arrangement and structure of motile cells, which typically have two unequal flagella. Among the water moulds, these are produced as asexual spores called zoospores, which capitalize on surface water (including precipitation on plant surfaces) for movement. They also produce sexual spores, called oospores, that are translucent double-walled spherical structures used to survive adverse environmental conditions. A few oomycetes produce aerial asexual spores that are distributed by wind.

The water moulds are economically and scientifically important because they are aggressive plant pathogens (see plant pathology). The majority can be broken down into three groups, although more exist.

• The Phytophthora group is a genus that causes diseases such as potato blight, sudden oak death and rhododendron root rot.
• The Pythium group is a genus that is more ubiquitous than Phytophythora and individual species have larger host ranges, usually causing less damage. Pythium damping off is a very common problem in greenhouses where the organism kills newly emerged seedlings.
• The third group is the downy mildews, which are easily identifable by the appearance of white "mildew" on leaf surfaces (although this group can be confused with the unrelated powdery mildews).