INTRODUCTIONDictyostelium discoideum is a unicellular eukaryote often referred to as a "social ameba" because it can form a multicellular structure when nutrient conditions are limiting. General principles for cell-to-cell communication, intracellular signaling, and cytoskeletal organization during cell motility have been derived from cellular and molecular studies of Dictyostelium and have been found to be conserved across all eukaryotes. Dictyostelium also provides an excellent model system for the study of phagocytosis, the molecular basis of various human diseases, and the mechanisms of drug action. The availability of a complete genome database and stocks of wild-type and mutant strains make D. discoideum an accessible and powerful model organism. Most Dictyostelium strains used in the laboratory can be grown either with bacteria or in axenic medium. When grown in the presence of bacteria, cells double approximately every 4 h, whereas axenically grown cells double more slowly, every 8-12 h. The cells can be grown in a standard microbiology incubator or on the laboratory bench, provided the room temperature is consistently ~22°C. This protocol describes a variety of methods for growing and maintaining Dictyostelium.