Police are increasingly viewed as front-line determinants of HIV risk among vulnerable populations. The negative impact of egregious police behaviors on HIV risk (e.g., sexual or physical violence, condom confiscation) has received more attention in the literature over recent years. However, the impact of routine law enforcement practices on HIV risk has not, and even less common are attempts to understand the drivers for those practices. Given the pressing need to engage police in the HIV response, examining the drivers for and impacts of policing on cis-gender (individuals assigned female at birth who identifies as a woman) female sex workers’ (hereafter FSW) HIV risk warrants attention. This thesis aims to examine the role of policing on FSWs’ HIV risk, with a particular focus on understanding the drivers behind policing approaches. The thesis is grounded in an initial systematic literature review, a study that sought to elucidate the extent of the quantitative literature exploring specifically policing as a determinant of HIV. Specifically, the study included 16 studies and found that the majority used a single measure to capture police behaviors, with studies predominantly focused on ‘extra-legal policing practices’. All studies found an association between police behaviors and either a HIV/STI outcome or related risk behavior, but amounted to a small body of evidence. The review pointed to a need for more studies focused specifically on police behaviors as structural determinant, in addition to development of more nuanced quantitative measurement. Based on the findings of the systematic literature review the remainder of the analysis via a ethnographic study, conducted collaboratively with Baltimore City Police Department and a quantitative cohort study with street-based FSW in Baltimore City, USA explores the nature and impact of policing on the HIV risk of FSW. The ethnographic study used 281 hours of 2 observational data in the form of police ride-outs completed over a 8 month period ( July 2015 - February 2016), to probe the question often missed by the literature: “Why do police do what they do?” Specifically, the study explored what factors influence police practices already identified in the literature as adversely affecting street-based FSWs’ HIV risk and human rights and police officers’ ability and willingness to adopt a more public health and rights orientated approach to street-based sex work. The ethnography highlights ecological factors at the structural (e.g., criminalization), organizational (e.g., violent crime control), community and individual level (e.g., stigmatizing attitudes) that are key to shaping individual police practices and attitudes towards FSW. The quantitative study draws upon baseline data collected over a 10-month period (April 2016 to January 2017) from 250 FSW from the Baltimore-based SAPPHIRE study. The study characterized FSW encounters with the police and explored the contribution that both day-to-day and abusive police practices may have on a risk environment that promotes client perpetrated violence. The study found that police interactions had a profoundly negative association with each additional type of abusive interaction being associated with client violence in adjusted analysis. While marginally non-significant, the association of the patrol-enforcement activities in adjusted analysis with client violence were of the same magnitude as the abusive interactions. These findings suggest that even nonabusive encounters, which occur on a much more frequent basis than abusive ones, also contribute to a risk environment that can facilitate client-perpetrated violence. Collectively the findings support existing calls for decriminalization of sex work, supported by institutional and policy reforms, neighborhood-level dialogues that shift the cultural landscape around sex work within both the police and larger community, and innovative individual-level police trainings to address abusive and coercive policing.