No general comprehensive theory of underachievment exists despite much research having been performed and hundreds of articles having been published. Further, underachievement as a concept often seems to be divorced from the individual pupil's or student's unique problem(s) as an underachiever. Indeed, even the designation "underachiever" differs according to research designs used to collect data. The practitioner desiring to understand and help underachievers will benefit from acquaintance with various aspects of the subject such as definitions, classification systems, research inadequacies, improvement possibilities, and bibliographies of published research. The purpose of this paper is to present the kind of general overview which will provide that acquaintance. (Author)