This introduction presents an overview of key concepts covered in the subsequent chapters of this book. Ideas about crime, policing, punishment and offenders have circulated within and between nations, throughout colonial empires and across post-colonial societies for centuries. Travellers' accounts, newspaper reports, pamphlets, journals, memoirs, official government records and the meetings of national and international prison congresses have provided spaces in which penal cultures have been interpreted, challenged, adapted, transformed and translated. Understandings and practices of criminality, policing and punishment have also travelled with and evolved through the work of law enforcement officers, lawyers, judges, prison wardens and administrators as they moved between different judicial and penal regimes, particularly in colonial contexts. They have been further shaped by defendants, offenders and convicts, whose encounters with criminal justice systems and penal institutions have often had global dimensions. Criminal operators and illegal activities also span often complex transnational and transcontinental networks.