© Bokhour et al. Background: Increasing the use of routine preventive care such as HIV testing is important, yet implementation of such evidence-based clinical care is complex. The Promoting Action on Research Implementation in Health Services (PARiHS) model for implementation posits that implementation will be most successful when the evidence, context, and facilitation strategies are strong for the clinical practice. We evaluated the relative importance of perceived evidence, context, and facilitation of HIV testing during the implementation of a multimodal intervention in US Department of Veterans Affairs primary care clinics. Methods: A multimodal intervention including clinical reminders (CRs), academic detailing-providing education sessions for providers-and social marketing to improve HIV testing was implemented in 15 VA primary care clinics in three regions. We conducted qualitative formative and process evaluations using semi-structured interviews with HIV lead clinicians, primary care lead clinicians, nurse managers, and social workers. Interviews were analyzed thematically to identify barriers and facilitators to implementation of HIV testing and how these were addressed by the intervention. Sites were then rated high, medium, or low on the dimensions of perceived evidence and the context for testing. We then assessed the relationship of these ratings to improvements in HIV testing rates found in earlier quantitative analyses. Results: Sites that showed greatest improvements in HIV testing rates also rated high on evidence and context. Conversely, sites that demonstrated the poorest improvements in testing rates rated low on both dimensions. Perceptions of evidence and several contextual aspects resulted in both barriers and facilitators to implementing testing. Evidence barriers included provider perceptions of evidence for routine testing as irrelevant to their population. Contextual barriers included clinical reminder overload, insufficient resources, onerous consent processes, stigma, provider discomfort, and concerns about linking individuals who test positive to HIV treatment. While most barriers were ameliorated by the intervention, HIV stigma in particular regions and concerns about linkage to care persisted. Conclusions: Interventions to implement evidence-based practices such as HIV testing can be successful when utilizing proven quality improvement techniques. However, it is critical to address providers' perceptions of evidence and consider aspects of the local context in order to fully implement new routine clinical practices such as HIV testing.