Synchronised neuronal oscillations at beta frequencies are prevalent in the human motor system, but their function is unclear. In this Opinion article, we propose that the levels of beta oscillations provide a measure of the likelihood that a new voluntary action will need to be actuated. Oscillatory beta activity is in turn modulated by net dopamine levels at sites of cortical input to the basal ganglia. We hypothesise that net dopamine levels are modulated in response to salient internal and external cues. Crucially, the resulting modulation of beta activity is predictive, enabling the appropriate prospective resourcing and preparation of potential actions. Loss of dopamine, as in Parkinson's disease, annuls this function, unless net dopaminergic activity can be elevated through medication.