Abstract: The 10 personality disorders are the paranoid, schizoid, and schizotypal; the histrionic, antisocial, borderline, and narcissistic; and the avoidant, dependent, and obsessive–compulsive. By definition, personality disorders must be evident since adolescence or young adulthood and have been relatively chronic and stable throughout adult life. As such, they often predate the occurrence of other mental disorders, such as a mood, anxiety, or substance use disorder. This chapter focuses on the assessment of personality disorders in adults. It begins with a review of the nature of the disorders, which is followed by a review of clinical assessment instruments designed for the assessment purposes of (a) diagnosis, (b) case conceptualization and treatment planning, and (c) treatment monitoring and evaluation. Recommendations are included for instruments with the greatest scientific support and for assessing personality disorders in a clinically sensitive manner.