Free will is the purported ability of agents to make choices free from constraints. Historically, the constraint of dominant concern has been the metaphysical constraint of determinism. The opposing positions within that debate are metaphysical libertarianism, the claim that determinism is false and thus that free will exists; and hard determinism, the claim that determinism is true and thus that free will does not exist.

Both of these positions, which agree that causal determination is the relevant factor in the question of free will, are classed as incompatibilists. Those who deny that determinism is relevant are classified as compatibilists, and offer various alternative explanations of what constraints are relevant, such as physical constraints (e.g. chains or imprisonment), social constraints (e.g. threat of punishment or censure), or psychological constraints (e.g. compulsions or phobias).

The principle of free will has religious, ethical, and scientific implications. For example, in the religious realm, free will implies that an omnipotent divinity does not assert its power over individual will and choices. In ethics, it may hold implications regarding whether individuals can be held morally accountable for their actions. The question of free will has been a central issue since the beginning of philosophical thought.