Antiretroviral (ARV)-based prevention approaches such as Treatment as Prevention (TasP) and Pre-Exposure Prophylaxis (PrEP), present unprecedented opportunities for stemming the HIV epidemic in the United States. However, current racial/ethnic disparities in both the PrEP and HIV Continuums of Care point to significant implementation problems and gaps in the diffusion and uptake of these interventions. Community-based organizations (CBOs) that offer HIV prevention programming are uniquely positioned to assist our nation’s most vulnerable populations with accessing TasP and PrEP, which includes navigating their options under the Affordable Care Act (ACA). However, these prevention approaches represent a shift in HIV prevention practice ideologies, which requires significant change in the way that CBOs have historically approached HIV prevention. The U.S. Centers for Disease Control and Prevention (CDC), which is the largest funder of HIV prevention services in the country, has long recognized CBOs as important partners in the domestic fight against HIV/AIDS. In response to changing trends in the epidemic over time, the CDC’s HIV prevention program with CBOs has grown in size, scope, and complexity. Little is known, however, about the capacity of CBOs to support the shift in practice ideology at the CDC that prioritizes ARV-based prevention, or about how local variations in ACA implementation impact the ability of organizational staff to integrate such approaches into their existing programming. Grounded in resource dependence and new institutional theories, this dissertation employed qualitative research methods and is organized around two primary aims. The first aim was to explore the impact of emerging ARV-based approaches on HIV prevention practice ideologies as they are communicated to CBOs from the CDC. To achieve this aim, I conducted a document analysis of Funding Opportunity Announcements (FOAs) and other reports issued by the CDC related to community-based HIV prevention between 2003 and 2013. The second aim was to examine—via semi-structured interviews with organizational leaders, frontline prevention workers, and public health officials in Atlanta, Baltimore, Memphis, and New Orleans—how a diverse mix of CBOs in four cities heavily impacted by HIV/AIDS and with differing ACA implementation strategies are responding to shifts in the CDC’s program for community-based HIV prevention. Two primary shifts in HIV prevention practice ideologies communicated to CBOs from the CDC are revealed through this analysis. First, in accordance with the Advancing HIV Prevention initiative announced in 2003, funded organizations were required to implement standardized evidence-based behavioral interventions as determined by the CDC—in contrast to locally developed models. Then, as antiretroviral-based prevention emerged and evolved over time, the CDC’s program for community-based HIV prevention shifted from a primary focus on behavioral interventions to the inclusion of these biomedical approaches. In addition to shifts in practice ideologies, the CDC has also increasingly acknowledged within its program for community-based prevention that disparities in HIV are driven by larger contextual factors, including social and economic inequalities. As such, the document analysis also revealed concurrent shifts in structural approaches to preventing HIV/AIDS, intended to address the role of social context as a compliment to individual-level outreach and intervention strategies. Such approaches included: (1) expanding community-based prevention efforts to include people living with HIV/AIDS (PLWHAs); (2) targeting prevention efforts towards disproportionately affected populations (e.g., gay/bisexual men, transgender women) in geographic regions where HIV prevalence is highest (e.g., the southern U.S.); and (3) addressing the social/economic determinants of health facing the most vulnerable populations (e.g., poverty, homelessness, unemployment, and racial discrimination). Given the significant degree of dependence on the CDC for HIV prevention resources that exists among the CBOs included in this study, these changes created a great deal of coercive and mimetic isomorphic pressure within the organizational field. Many CBOs staff changed their organizational infrastructures and focus populations to comply with changes at the CDC, and/or engaged in loose or de-coupling strategies to appear to be in compliance with funding mandates, at times neglecting the true HIV prevention needs of the communities that they serve. And, while CBO staff overwhelmingly agree with and support the biomedical HIV prevention practice ideology, without additional resources and recommendations for evidence-based models for addressing the social and economic determinants of health, access to ARV-based prevention remains limited among the most vulnerable populations.