Slime molds were once placed in the kingdom Fungi, although some authorities now place them in the kingdom Protoctista (Protista). Most fungi are composed of threadlike filaments of eukaryotic cells called hyphae, collectively referred to as a mycelium. Slime molds are composed of an acellular mass of naked protoplasm with no cell walls in its vegetative state. They are typically found in damp, shady areas with abundant organic matter, although they may move to bright areas to "fruit."

The vegetative stage is essentially a multinucleate mass of protoplasm (called a plasmodium) composed of numerous diploid nuclei. Depending on the species, the plasmodium may be only a few millimeters in diameter to large pizza-sized slime molds over 12 inches (30 cm) across. The plasmodium moves like a giant amoeba, flowing over the surface as it ingests organic matter, such as dead leaves and wood. It may be be brightly colored, such as the yellow Physarum polycephalum commonly studied in general botany laboratories. Some tropical slime molds are bioluminescent and glow in the dark. Slime molds are important heterotrophs in the decomposition of dead organic matter in temperate and tropical forests.