This review focuses on selected issues in the rapidly proliferating scientific literature on attention deficit disorder (ADD). It encompasses a brief overview of ADD, including a review of the historical trends, causation, and a description of the clinical characteristics. Critical issues fundamental to understanding of ADD are addressed next. Evidence is presented to suggest that there are several groups of children with ADD who are currently under-identified and therefore, underserved (girls with ADD; ADD without hyperactivity). The concept is introduced of uncomplicated ADD and ADD-Plus (ADD-P), a term used to designate ADD associated with other complicating features such as conduct disorder. Specific methodologic issues and their relationship to the under-identification of these groups of children follows. This discussion encompasses the reasons for diagnostic imprecision and inconsistency, emphasizing the problem of selective referral patterns and how it results in an inaccurate portrait of ADD. We then present new approaches to the definition and diagnosis of ADD based on empiric studies designed to circumvent some of these problems. We conclude with a summary of the implications of these findings, and recommendations for public policy in ADD, particularly the need to recognize that ADD may occur even in the most intellectually gifted individuals and even in students in select colleges. Often these children with the highest potential are penalized most, not because of conceptual limitations or because they do not understand, but because educators often fail to recognize the symptom complex. Many of these children are succeeding through intelligence and great effort, but many more could succeed with proper identification.