This chapter explores the nature of psychopathy, its key features and how it is measured in both forensic and general populations. It examines the relationship between psychopathy and other disorders, as well as its relationship with criminal behaviour, with a particular focus on its prevalence in sexual offending. This chapter will also discuss the history of psychopathy, relevant theories and models and the potential existence of ‘successful psychopathy’. By the end of this chapter, you will understand psychopathy’s relevance to forensic psychology and the on-going debates and discussions in this field.