Repeated intense skeletal muscle contraction leads to a progressive loss of force-generating capacity. This decline in function is generally referred to as muscular fatigue. Since the work of Fletcher and Hopkins (1907), it has been known that fatigued muscles accumulate lactic acid. Intracellular acidosis due to lactic acid accumulation has been regarded as the most important cause of fatigue during intense exercise. Recent challenges to the traditional view, however, have suggested that lactic acid plays a role in muscle contraction distinct from that implied by earlier studies. This brief review presents (1) a short history of our understanding about lactic acid that explains the early acceptance of a causal relationship between lactic acid and fatigue, (2) evidence to show the temperature dependence of acidosis-induced changes and the beneficial effect of acidosis, and (3) a proposal that lactate production retards, not causes, acidosis. These findings require us to reevaluate our notions of lactic acid, acidosis and muscular fatigue.