19 We briefly summarize the literature that provides two possible conceptual explanations for clustering: ( 1) cost-reduction advantages associated with localization and urbanization economies and ( 2) lower levels of law enforcement monitoring associated with moderate- and higher-income neighborhoods. 21 The apparent growth in the indoor sex work industry has been traced in part to more aggressive policing of street-based sex workers, driving sex work into indoor venues, and use of the Internet to connect clients and sex workers. The indoor sex industry in the United States ranges from individual workers providing services in their homes to associated businesses where workers provide a combination of legitimate massage services and illegal sex services. 15, 19 One study has suggested, for example, that sex work outside of red-light districts is associated with increased pressure on female sex workers to not use condoms. 16– 18 Few studies have focused on indoor sex work businesses outside of red-light districts, leaving gaps in knowledge about health for female sex workers and their male clients. 15 In studying spatial aspects of outdoor sex work, researchers have focused on “red-light” districts, where street-based sex workers congregate and “adult use” businesses (e.g., porn shops, strip clubs) cluster. Although some studies suggest that indoor sex work has become the most prevalent form of sex work in the United States, 12, 13 constituting up to 85% of all sex work activity, 14 most research has focused on street or “outdoor” sex work. Sexually oriented massage parlors are categorized as “indoor sex work” 11 because, unlike street-based sex work, transactions occur indoors. 9 Lower condom use rates in this population are associated with lack of support for condom use at venues and financial incentives from clients (e.g., a client offering money to not use a condom). 7 Sexually oriented massage parlors in the United States tend to employ mostly immigrant Asian as well as Latina/Hispanic women, 8 populations experiencing substantial health disparities. Sex workers in these businesses are at risk for sexually transmitted infections, physical violence, and emotional abuse. We examined the spatial clustering of sexually oriented massage parlors, a business type associated with negative health outcomes, in Southern California. 5, 6 What is less clear, however, is why such businesses cluster in specific locations. Recent studies have investigated causal factors associated with clustering (e.g., the clustering of liquor stores associated with poverty 3, 4 and child maltreatment). One important spatial aspect is clustering. 1 In developing new, more “upstream” interventions, 2 which target earlier points in the causal chain, it is necessary to develop a greater understanding of the intersection between social determinants and the places in which inequities persist. The public health field is increasingly aware that social determinants are intertwined with place (i.e., the spatial determinants of health). Social determinants are recognized as important for population health, especially for the most vulnerable populations.