This chapter provides an in-depth historical analysis of prostitution markets in Paris during the 19th century. More specifically, it explores the economic rationalities of the different actors in the informal public prostitution network and how their behavior affects the financial considerations of the other actors in the urban economy. Before discussing the economics of popular prostitutions in fin-de-siècle Paris, the chapter takes a look at streetwalkers and their role in the local economy. It then considers the supply and demand for street prostitutes in Paris, along with the negative externalities of public prostitution in the city. In particular, it examines the impact of street prostitution on regulated brothels, shopkeepers, and annuitants. It also takes into account the positive externalities of street prostitution in relation to wine merchants and slumlords and concludes with an assessment of the red-light district of fin-de-siècle Paris.