This chapter introduces the black market sex industry in late colonial Vietnam. It argues that the interwar black market sex industry thrived in spaces of tension, which were created by the confluence of economic, demographic, and cultural changes sweeping late colonial Tonkin. Tension developed in sites of legal inconsistency, cultural changes, economic disparity, rural–urban division, and demographic shifts, brought about by colonial policies. An investigation of this black market shows how a particular population of impoverished women — a group regrettably understudied by historians — experienced the tensions. Marginalized by the colonial economy and swayed by new cultural trends, these women came to participate in black market sex work by choice, by force, or, more often, by some combination of the two.