This Element explores the conceptual complexity of time reversal in the philosophy of physics. It aims to show that time reversal, as a symmetry transformation, should not be regarded as a mere mathematical artifice applied to physical equations. It is rather a conceptually rich and multifaceted notion, one whose meaning and implementation are shaped by a combination of metaphysical commitments and heuristic-methodological strategies. Far from being a neutral tool, the way we define and apply time reversal encodes assumptions about the nature of time itself, its relation to motion, about the role of symmetries in physical theories, and about the relation between mathematical symmetries and the world they purport to describe. Such conceptual complexity also has implication for related debates, such as that of the direction of time.
