The authors review the literature on the diagnosis of borderline personality disorder and describe the core dynamic features that account for its unstable phenomenology and explain in part the complexity of and the controversies associated with the diagnosis. A review of differential diagnosis suggests that the boundary with schizophrenia is well established, that the boundary with affective disorder is becoming clearer, and that the most subtle boundary distinctions lie with other forms of personality disorder. A growing body of evidence about antecedents to and longitudinal aspects of borderline personality disorder confirms its diagnostic validity.