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. D. discoideum and related organisms, known as the Dictyostelia, have been studied for almost 150 years. The cellular and molecular aspects of their multicellular lifestyle have been studied in detail, and general principles for cell-to-cell communication, intracellular signaling, and cytoskeletal organization during cell motility have been derived from this work and have been found to be conserved across all eukaryotes. The bacteriovore nature of the unicellular stage provides an excellent model in which to study phagocytosis and the mechanisms of bacterial virulence. D. discoideum has also been used successfully to explore the molecular basis of various human diseases, as well as the mechanisms of drug action and the pathways that lead to resistance to certain therapeutic agents. The availability of a complete genome sequence has further widened the scope of studies using D. discoideum. A large potential for secondary metabolism has become apparent, which opens the door to discovering new compounds with potential medical applications. Numerous putative orthologs of genes responsible for diseases in humans, but whose molecular functions are still uncharacterized, are present in the D. discoideum genome. Finally, the availability of community resources, including the genome database dictyBase and the Dicty Stock Center, makes D. discoideum an easily accessible and powerful model organism to study.