In this chapter, working with scientific evidence, I build up a picture of the psychopathic personality which can be applied to my preferred account of moral responsibility. After sketching an introduction to the history of psychopathy as a clinical construct, I consider some disputes and controversies surrounding its diagnosis. I distinguish psychopathy from Antisocial Personality Disorder (APD), a rival construct commonly used in clinical settings. I also sketch the implications of evidence for a distinction between ‘successful’ and ‘unsuccessful’ psychopaths for the overall construct. I conclude that the Hare Psychopathy Checklist is the most robust measure available, a measure which describes psychopathy as a condition characterised primarily by emotional deficiencies. I then review neuroscientific evidence for structural and functional correlates and causes of psychopathy. I also review evidence for the treatability of the condition, concluding based on the current psychological and psychiatric evidence that psychopathy appears to be highly recalcitrant to the treatment methods that have been tried so far, and that some of these methods may even be counter-productive.