What is the function of our capacity for 'mental time travel'? Evolutionary considerations suggest that vivid memory and imaginative foresight may be crucial cognitive devices for human decision making. Our emotional engagement with past or future events gives them great motivational force, which may counter a natural tendency towards time discounting and impulsive, opportunistic behavior. In this view, whereas simple episodic memory provides us with a store of relevant, case-based information to guide decisions, mental time travel nudges us towards more restrained choices, which in the long term are advantageous, especially so given human dependence on cooperation and coordination.