This book provides a transcription of the reports written by undercover agent Paul Kinsie for the League of Nations Special Body of Experts on Traffic in Women and Children in the mid-1920s. Between 1924 and 1926, a team travelled to more than a hundred cities in Europe, the Americas and the Mediterranean area to interview individuals involved in the regulation, repression, medical control, organization and practice of the sex trade. American undercover agents were included on the team to infiltrate the so-called ‘underworld’ and obtain ‘facts’ about the traffic. Among these, Kinsie was the most prolific. He visited more than forty cities and produced hundreds of reports in which his contacts with prostitutes, brothel owners, madams, pimps and procurers are described in detail. For a proper contextualization of the reports, scholars from around the world were asked to provide short introductions to the situation with regard to prostitution in each city that was visited. The book offers a unique source of information which is of great ethnographic value for people interested in the history of human trafficking and prostitution.