Biological research is in an explosive phase, driven by a new awareness of the basic unity of all life forms. Today, the key to a problem about neurons, plant cells, or cancer may lie in research on yeasts, frogs, or flies. More than ever before, molecular genetics is showing us how to recognize and exploit such connections and is forcing us to reflect on the ancient origins of the components from which we are made. In surveying cell biology, one does not know whether to marvel more at the endless variety of living systems or at the fundamental similarities in the mechanisms by which all cells operate.