One of the most widely cited models in time estimation is the " pacemaker-counter clock " which consists of a pacemaker generating pulses and an accumulator in which pulses are stored. The level reached in this accumulator at the end of an interval to be estimated sets the subjective elapsed time. Although this model is able to accurately describe temporal performance, it can nevertheless be considered no more than a good metaphor of actual temporal processing mechanisms. Finding specific brain areas behaving as a pacemaker and/or as an accumulator would provide additional support for this model. Evidence has now been amassed suggesting that the SMAs could be a neural substrate of the accumulator.