This article deals with the rationality and functionality of the existence of regret and its influence on decision making. First, regret is defined as a negative, cognitively based emotion that we experience when realizing or imagining that our present situation would have been better had we acted differently. Next, it is discussed whether this experience can be considered rational and it is argued that rationality only applies to what we do with our regrets, not to the experience itself. Then, research is reviewed showing that both the anticipation of future regret and the experience of retrospective regret influence behavior. The influence of anticipated regret can be considered rational as long as the decision maker can accurately predict the regret that may result from the decision. The influence of experienced regret cannot be considered rational, since decisions should be based on future outcomes, not historical ones. However, influence of experienced regret can be called functional since it may result in increased learning from our mistakes.