This NIJ funded study sought to measure the size and structure of the underground commercial sex economy in eight major US cities. The study was guided by four main research questions: 1) How large is the underground commercial sex economy in eight major U.S. cities?; 2) To what extent are the underground commercial sex, drug and weapons economies interconnected in the eight major U.S. cities?; 3) How do the ties between traffickers within the underground commercial sex economy affect the transportation of sex trafficking victims?; and 4) What are the network characteristics of the traffickers that operate within the underground commercial sex economy? The study found that the estimates of the size of the underground commercial sex economy in the eight cities studied ranged from $39.9 million to $290 million in 2007. In five of the cities, the size decreased between 2003 and 2007. Researchers did not find a connection between weapons trafficking and the underground commercial sex economy, but in some cities, there was an overlap with drug trafficking. In five of the study sites, gang involvement in sex trafficking and prostitution seemed to be increasing.