Slime molds are inconspicuous organisms commonly found in soil or on rotting logs. There are several groups that share the characteristic of phagotropic nutrition (i.e., engulfment of food particles), during vegetative growth. The vegetative growth phase in some of the slime molds consists of non-flagellated, free-living cells. In others, the vegetative phase consists of large mulinucleate protoplasm called a plasmodium (Fig.1). The two groups are called cellular and plasmodial slime molds respectively. The vegetative phase in both groups is more animal-like than plant-like. Some taxonomists therefore classify them as protozoans. However, these organisms are also capable of producing spores in walled sporangia, a more mold-like characteristic. Sporangia in plasmodial slime molds are formed from an outgrowth of the plasmodium (Fig.2). In cellular forms, sporgangia formation is preceded by loose aggregation of free-living cells. The aggregated cell mass is called a false or "pseudo-plasmodium."

Spores in plasmodial slime molds are the products of meiosis. They germinate to produce either amoeba-like cells or flagellated gametes. Fusion either of the gametes or of the amoeba-like cells re-establishes the diploid plasmodial phase.

There are approximately 750 known species of slime molds. At present, there are no verifiable fossils. As a result, phylogenetic origin is very speculative. A number of algal, fungal and protozoan groups have been suggested as possible parent stock. The various groups of slime molds are probably not directly related to each other.

Slime molds are of little economic importance. They do serve, however, as important research organisms. Aggregation and differentiation of cells in pseudoplasmodia of cellular slime molds, for example, is a model system for the study of cell communication.