The capacity to inhibit and withhold actions is a key feature of human cognition. Withholding action forms the basis of self-control, delayed gratification, social contracts, and trust in others. Most experimental studies of this function come from studying the processing of external stop signals. However, another important aspect of inhibition is ‘will-power’, i.e., intentional inhibitory control over one's own actions, in the absence of external countermanding signals. We review whether a concept of intentional inhibition is justified, and how it might differ from externally triggered inhibition. Further, we consider three types of neuroscientific evidence that can clarify the brain's mechanisms of inhibition: neuropsychology, neurostimulation and neuroimaging. Finally, we propose a model in which intentional inhibition, unlike externally triggered inhibition, is linked to representing longer range consequences of action decisions. We suggest that the human brain contains a ‘neural brake’ mechanism that blocks specific ongoing motor activity, and that this mechanism plays a key role in action decisions.