This chapter concerns differences and similarities in the psychological research on personality traits and the psychiatric research on personality disorders. There are striking similarities in assumptions about the stability and consequences of traits and disorders. While the psychological research on the classification of traits has been dominated by psychometric work, the psychiatric world of disorder classification has shown relatively few changes over 50 years. There has, however, been something of a rapprochement between the two, particularly with personality psychologists being interested in subclinical disorders now known as dark-side traits, in contrast to the bright-side traits they have always investigated. There is now also a rapidly growing literature on the Dark Triad and Dark Tetrad, which focuses on three or four disorders (Machiavellianism, narcissism, psychopathy, sadism). This chapter also concerns those interested in leadership at work success and failure who have turned to the dark side to offer insights into derailment.