Although most researchers state that emotion is a very difficult concept to define, what is now being called a functionalist approach to emotion proposes that emotion is very easy to define. Emotion occurs whenever an event is encountered which is significant to the person. There is no exception to this rule. What makes an event significant is the RELATION between an event and one of four fundamental axioms: 1) how the event relates to a person's goals, 2) how an event links up with the hedonic state of the person, 3) how an event relates to the social reactions of another person, and 4) how an event relates to the prior experiences of an individual, such as in the attachment relationship. In this article, we intend to propose cogently some the basic principles of a functionalist approach, how it differs from the orthodox approach to emotion, how it is relevant to the contemporary interest in emotion regulation, and how it can be illustrated in experiments in infancy.