The paper uses John L. Holland's theory of careers to explain certain common career phenomena and concepts. An understanding of careers requires useful answers for four fundamental questions relating to vocational choice, vocational stability and instability, and vocational congruence. The theory of careers attempts to answer these fundamental questions by using a few carefully defined concepts (such as theoretical personality types and environmental models) to explain vocational behavior. A modern differentialist view can better reinterpret some common vocational developmental concepts, such as those associated with vocational maturity, personal integration, identity, crystalization of interests, and vocational adjustment than a developmental view. The typology of persons and environments is more useful than any of the life stage strategies in formulating a theory of careers applicable to the entire life span. It also is suitable for assessing career changes and crises, as well as for explaining the career experiences of women and minority groups. In summary, the typology can be used to organize occupational materials and experiences, to explain and interpret vocational data and behavior, and to plan remedial activities. It can serve as the basis for an entire vocational assistance orientation including self-use materials and, where needed, personal and group counseling. (Author/JR)