Personality, essentially, refers to who a person is and how that person behaves. It influences an individual’s thoughts, feelings, attitudes, values, motivations, and behaviors. Personality disorders are classified into clusters A, B, or C based on the predominant symptoms. Cluster A personality disorders include: paranoid, schizoid, and schizotypal personality disorders. Cluster B personality disorders include: antisocial, borderline, histrionic, and narcissistic personality disorder. Cluster C personality disorders include: avoidant, dependent, and obsessive-compulsive personality disorders. This chapter addresses the historical perspectives and epidemiology of personality disorders as defined in the Diagnostic and Statistical Manual of Mental Disorders (DSM-5). It also addresses current psychosocial and biological etiological influences of personality disorders along with current treatment modalities. The chapter presents application of the nursing process from an interpersonal perspective, including a nursing plan of care for a patient with a personality disorder.