The paper traces the vicissitudes of the Yerkes-Dodson law from 1908 to the present. In its original form, the law was intended to describe the relation between stimulus strength and habit-formation for tasks varying in discrimination difficultness. But later generations of investigations and textbook authors have rendered it variously as the effects of punishment, reward, motivation, drive, arousal, anxiety, tension or stress upon learning, performance, problem-solving, coping or memory; while the task variable has been commonly referred to as difficulty, complexity or novelty, when it is not omitted altogether. These changes are seldom explicitly discussed, and are often misattributed to Yerkes and Dodson themselves. The various reformulations are seen as reflecting conceptual changes and current developments in the areas of learning, motivation and emotion, and it is argued that the plasticity of the law also reflects the vagueness of basic psychological concepts in these areas.