Borderline personality disorder (BPD) is a complex and serious mental disorder associated with severe functional impairment, substantial treatment utilization, and a high rate of mortality by suicide. Recently, BPD has become a focus of intensifying study. In Part I of this three-part article meant to provide a foundation to researchers on the current status of the borderline diagnosis and prospects for its future development, we examine the psychopathology, comorbidity, and personality structure of BPD. Although the descriptive characteristics of BPD are well-represented by DSM-IV diagnostic criteria, other important aspects of BPD psychopathology are not included. The descriptive criteria in conjunction with semistructured interviews have, however, increased the ability of investigators to diagnose BPD as reliably as many Axis I disorders. Frequent comorbidity of BPD with Axis I disorders necessitates a broad assessment of psychopathology to help account for clinical heterogeneity. Because of the absence of evidence of the validity of the diagnostic threshold for a categorical diagnosis of BPD, and because of the heterogeneity within the diagnosis, investigators should also supplement their DSM-IV diagnoses with assessments of underlying personality trait structures. Although there are a number of competing models of personality structure, they have remarkable convergence on a set of three to five basic personality dimensions.