K0 describes a point of view on maturity that departs from earlier treatments in two ways. First, it rejects the popular assumption from humanistic psychology that maturity is a function of self-actualization and stipulates that maturity is related to certain performance capacities--namely, the ability to form lasting relationships and to achieve one's career goals. Second, the article is based on an explicit model of personality, and the model holds that personality is most productively viewed from the perspective of the actor and of the observer. This means that maturity must also be defined from two perspectives--how people feel about themselves and how others feel about them. The authors briefly review some data bearing on these observations.