Here we talk about mechanisms of organic reactions, that is, a physical model describing how particular organic molecules collide, interact, and react. The first chemist to discuss this in a modern way was Arthur Lapworth. We move to Robert Robinson, Arthur Michael, and then Irving Langmuir. Finally the current idea of a reaction mechanism was given by Christopher Ingold in the 1920s and 1930s. We talk of molecular symmetry and its relationship to reaction mechanisms.