Dictyostelium discoideum is a simple but fascinating eukaryotic microorganism, whose natural habitat is deciduous forest soil and decaying leaves, where the amoebae feed on bacteria and grow as independent single cells. Exhaustion of the bacterial food source triggers a developmental program, in which up to 100,000 cells aggregate by chemotaxis towards cAMP. Morphogenesis and cell differentiation then culminate in the production of spores enabling the organism to survive unfavorable conditions. Dictyostelium offers unique advantages for studying fundamental cellular processes with the aid of powerful molecular genetic, biochemical, and cell biological tools. These processes include signal transduction, chemotaxis, cell motility, cytokinesis, phagocytosis, and aspects of development such as cell sorting, pattern formation, and cell type differentiation.