In 2010, the National Institute of Justice funded the Urban Institute's Justice Policy Center to measure the size and structure of the underground commercial sex economy in eight major US cities. The goals of this study were to: (1) derive a more rigorous estimate of the underground commercial sex economy (UCSE) in eight major US cities and (2) provide an understanding of the structure of this underground economy. To date, no reliable data exist to provide national or state policymakers with a verifiable and detailed understanding of underground commercial sex trade networks or the ways in which these networks interact with one another on the local, state, or interstate level. In addition, there is no information regarding the relationship between the UCSE and the local commercial sex trade or commercial sex activity conducted over the Internet. This study aimed to close the gap in our understanding about the nature and extent of these activities.