Every person has a characteristic manner of thinking, feeling, and relating to others. Some of these personality traits can be so dysfunctional as to warrant a diagnosis of personality disorder. The World Health Organization's International Classification of Diseases (ICD- 10) includes ten personality disorder diagnoses. Three issues of particular importance for the diagnosis of personality disorders are their differentiation from other mental disorders, from general personality functioning, and from each other. Each of these issues is discussed in turn, and it is suggested that personality disorders are more accurately and effectively diagnosed as maladaptive variants of common personality traits.