This chapter describes interest as having two meanings: It is both a psychological state and a motivational variable that can develop and it is noteworthy that throughout history interest has been recognized as a powerful influence on motivation and engagement. It focuses on one or the other conceptualization of interest did not diminish recognition of its impact and concerns on interest and its development beginning with the initial triggering of attention and extending through to the formation of a well-developed individual interest. Interest as a psychological state is grounded in a person's physiological/neurological reactions to a wide range of things, including other people, objects, and tasks. Interest describes a unique, content-specific, motivational variable that is responsible for the processes underlying how people act, feel, engage, and learn. The psychological studies of interest focus on one or another particular aspect of interest including interest as emotion, interest as influenced by the experience of interest, interest as value, or vocational interest.