Free will is the notional capacity or ability to choose between different possible courses of action unimpeded.

Free will is closely linked to the concepts of moral responsibility, praise, culpability, sin, and other judgements which apply only to actions that are freely chosen. It is also connected with the concepts of advice, persuasion, deliberation, and prohibition. Traditionally, only actions that are freely willed are seen as deserving credit or blame. Whether free will exists, what it is and the implications of whether it exists or not are some of the longest running debates of philosophy and religion. Some conceive of free will as the ability to act beyond the limits of external influences or wishes.

Some conceive free will to be the capacity to make choices undetermined by past events. Determinism suggests that only one course of events is possible, which is inconsistent with a libertarian model of free will. Ancient Greek philosophy identified this issue, which remains a major focus of philosophical debate. The view that conceives free will as incompatible with determinism is called incompatibilism and encompasses both metaphysical libertarianism (the claim that determinism is false and thus free will is at least possible) and hard determinism (the claim that determinism is true and thus free will is not possible). Incompatibilism also encompasses hard incompatibilism, which holds not only determinism but also indeterminism to be incompatible with free will and thus free will to be impossible whatever the case may be regarding determinism.

In contrast, compatibilists hold that free will is compatible with determinism. Some compatibilists even hold that determinism is necessary for free will, arguing that choice involves preference for one course of action over another, requiring a sense of how choices will turn out. Compatibilists thus consider the debate between libertarians and hard determinists over free will vs. determinism a false dilemma. Different compatibilists offer very different definitions of what "free will" means and consequently find different types of constraints to be relevant to the issue. Classical compatibilists considered free will nothing more than freedom of action, considering one free of will simply if, had one counterfactually wanted to do otherwise, one could have done otherwise without physical impediment. Contemporary compatibilists instead identify free will as a psychological capacity, such as to direct one's behavior in a way responsive to reason, and there are still further different conceptions of free will, each with their own concerns, sharing only the common feature of not finding the possibility of determinism a threat to the possibility of free will.