The possible ways in which a substance or technique can be regarded as "ergogenic" are first listed, and then a systematic account of the evidence that melatonin fulfils the requirements of such roles is undertaken. There is no evidence to indicate that the soporific and hypothermic effects of melatonin cause a decrease in physical performance, though its ability to decrease alertness and cognitive performance has been well established. The hypothermic effects of melatonin might, on the other hand, increase physical performance in two ways. First, they might allow more exercise to be performed before a "critical" temperature that causes the central nervous system to inhibit physical activity is reached. Second, the cutaneous vasodilatation produced by melatonin might promote heat loss during steady-state exercise and so guard against the rise of core temperature. Evidence for either of these roles is currently inadequate and insufficient for a confident judgement to be made. When the role of melatonin in promoting recovery from exercise is addressed, again, the data are too sparse and have often been obtained upon animals rather than humans. This is especially so with respect to the notion that melatonin might change favourably the metabolic responses to exercise. The general conclusion is that the case for/against melatonin as having a substantial ergogenic role has still to be made, and that far more evidence will be required before this can be done conclusively.