AbstractThe patient population that presents with both a B-cluster personality disorder and substance abuse is a challenge to professionals in both the mental health and substance abuse treatment fields. The coexistence of both disorders is not uncommon and is frequently encountered in substance abuse treatment, social welfare, and mental health settings. The complexity of the clinical picture for these patients and their greater resistance to treatment severely taxes the practitioner's skills. Even without a clear understanding of the primary causative pathology, effective treatment modes based on a recognition of the interactive and interdependent nature of the two disorders can be developed. Part I of this two-part article presents the characteristics and clinical features of the population as well as a model for understanding some of the motivating principles behind the patient's conduct, feelings, thoughts. This is followed by guidelines for the clinician's attitude, desired characteristics, and role in dealing with this population. Part II presents guidelines for the treatment of this patient population.