This paper discusses under an energetic perspective the recent and older evidence supporting the classical notion that the 'oxygen debt', as originally defined by Margaria et al. (1933) [Am. J. Physiol. 106, 689-714], consists of two major components: the alactic oxygen debt, with a half-time of the order of 30 sec, and the lactic oxygen debt, with a much longer half-time, similar to that of lactic acid removal from blood after exercise (approximately 15 min). In particular, two ensuing concepts are treated, namely (i) the energetic equivalent of blood lactate accumulation in blood, whence the notions of lactic power and lactic capacity, and (ii) the energy sources allowing contraction of the oxygen deficit at the onset of square-wave exercise. The notion of alactic oxygen deficit is rediscussed on the basis of recent evidence in humans. The analogies between lactate accumulation during supramaximal exercise and during exercise transients are discussed under an energetic perspective.