A simple dictionary definition of a motor is “something, such as a machine or an engine that produces or imparts motion”. This definition works well in the macroscale world that we normally interact with. However, at the molecular scale, there is an immediate problem with this definition — the influence of thermal or Brownian motion. Brownian motion is random and can hardly be seen as motor activity; although, as we detail below it makes an important contribution to how a molecular motor works. The influence that Brownian motion has on the way a molecular motor works is elegantly summarised by Oster and Wang7 with the phrase “In such an environment you would not need to even pedal your bicycle: you would simply attach a ratchet to the wheel preventing it from going backwards and shake yourself forwards!”. Therefore, we should rewrite the definition in a way that eliminates the problem of thermal motion and perhaps the simplest way is “something, such as a machine or an engine that produces or imparts directional motion”. Before discussing the wide variety of molecular motors that have been chemically synthesised, or exist in nature, it is important to describe the ways that motor activity can occur and discuss some of the problems molecular motors must overcome.