How common are personality problems? The honest, if unsatisfactory, answer is: it depends on your definition. Whether most of us have personality problems, or only relatively few of us, is determined by the context in which the problems are measured. In this chapter, we will attempt to be transparent about how personality problems are defined and measured, and the influence this may have on their frequency. But while acknowledging this, there is more consistency in the epidemiology of personality disorder than in many other aspects of the subject.