Treating Patients with Two or More Personality Disorders: An Introduction m Giancarlo Dimaggio Terzo Centro di Psicoterapia Cognitiva, Rome m John C. Norcross University of Scranton A high percentage of patients diagnosed with one personality disorder (PD) have, at the same time, at least one other PD. However, psychotherapists lack clinical guidance and research evidence on treating such complex patients. This issue of the Journal of Clinical Psychology: In Session attempts to bridge the gap. In this Introduc- tion, the editors outline the subsequent articles in this issue and address the terminology, prevalence, and possible causes of Axis II comorbidity. They argue that comorbidity is, in part, ascribable to problems in classification systems and can be reduced by defining PDs in a more multifaceted manner. They provide several clinical vignettes to portray frequent difficulties encountered by psychothera- pists in treating these complex, co-occurrent cases. & 2008 Wiley Periodicals, Inc. J Clin Psychol: In Session 64: 127–138, 2008. Keywords: personality disorders; comorbidity; Axis II; psychotherapy Treating patients suffering from personality disorders (PDs) is notoriously challenging because of the difficulties they typically experience in building a reliable therapeutic alliance, cooperating in problem solving, and reasoning in psychological terms. Psychotherapists have benefited from treatment manuals and models that describe the processes motivating patients’ dysfunctional behavior and that help them, as therapists, to master the emotional impacts that such patients frequently generate in them. For one prominent example, psychotherapists have been guided and comforted by demonstrably effective psychotherapies for patients suffering from borderline PD, including dialectical behavior therapy (Linehan, 1993), mentalization-based treatment (Bateman & Fonagy, 2004), transference-focused therapy (Clarkin, Yeomans, & Kernberg, 2006), and schema focused therapy (Young, Klosko, & Weishaar, 2003). Correspondence concerning this article should be addressed to: Giancarlo Dimaggio, Terzo Centro di Psicoterapia Cognitiva, via Ravenna 9/c 00161, Rome, Italy; e-mail: gdimaje@libero.it JOURNAL OF CLINICAL PSYCHOLOGY: IN SESSION, Vol. 64(2), 127--138 (2008) & 2008 Wiley Periodicals, Inc. Published online 9 January 2008 in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/jclp.20447