Personality consists of ingrained, pervasive, enduring and habitual ways of psychological functioning that characterize one’s style (Millon, 1987). Personality traits are defined as enduring patterns of perceiving; relating and thinking about the environment and oneself that are exhibited in a wide range of important social and personal contexts. Personality disorders according to ICD-10 guidelines “deeply ingrained and enduring behavior patterns, manifesting themselves as inflexible responses to a broad range of personal social situations. They represent either extreme of significant deviations from the way the average individual in a given culture perceives, thinks, feel and particularly relates to others. This chapter covers the significance of personality disorders, models of personality, classification of personality disorders, assessment of personality disorders, schizoid personality disorder, schizotypal personality disorder, avoidant personality disorder, antisocial personality disorder, borderline personality disorder, and histrionic personality disorder.