The concept of personality disorder has existed throughout the history of psychology and psychiatry. The ten current personality disorders (PDs) listed in the DSM-IV are conceived of as distinct syndromes. Nevertheless, DSM-IV PDs overlap extensively with each other, with normal personality traits, and with Axis I psychopathology. Thus, many investigators suggest that, in DSM-5, the classification system for PDs should be revised to better reflect the closeness of the links between personality and psychopathology, perhaps by linking both constructs within broader spectrums of psychopathological variation. The most common and well-accepted suggestion is that the categorical diagnoses of DSM-IV should be replaced with a dimensional model of pathological personality traits, and the links between personality traits and more syndromal forms of psychopathology (Axis I disorders) should also be explicitly articulated. In this chapter, we review current research on how PDs are linked with broad spectrums of personality and psychopathology. Specifically, we examine (1) evidence relevant to the etiology and course of PDs, (2) how these disorders can best be assessed, and (3) evidence regarding the most effective practices in the treatment of PDs, with the aim of articulating how etiology, course, assessment, and treatment of PDs might be conceptualized in a spectrum-based classification system.