A theory is outlined that assumes that emotions are motivational states with the special function of producing adaptation to situational conditions. The theory assumes that the emotional system lies in the central nervous system, that it is fast to react, able to change quickly from one emotional state to another, produces only one emotion at a time, and that the intensity of that emotion is a nonmonotonic function of deterrence to the aim of the emotion. Supporting data from several experimental tests are reported, and selected theoretical problems are discussed.