Background: Pre-exposure prophylaxis (PrEP) is key to ending the HIV epidemic in Taiwan, where men-who-have-sex-with-men (MSM) contribute to 79% of annual new infections. Community-based sexually-transmitted-infection-friendly physicians (CSTIPs) practise outside hospitals and are certificated by the Taiwan Centers for Disease Control to provide patients with treatment and prevention of sexually transmitted infections. CSTIPs have potential to deliver optimal decentralised PrEP services to MSM, but only two per cent of them prescribed PrEP in 2019. This thesis aims to develop a theory-informed questionnaire to measure the unmet needs of CSTIPs for delivering HIV PrEP services in Taiwan. Methods: This pragmatic qualitative research comprises two sequential phases as formative research and questionnaire development, applying a combination of the capability–opportunity–motivation–behaviour (COM-B) model and the Theoretical Domains Framework(TDF). The Formative Phase applied document research and 16 face-to-face in-depth interviews with CSTIPs in Taiwan. The diversity of interviewees was ensured through the purposive sampling of experience in PrEP prescribing, medical speciality and geography. I analysed the data using the COM-B/TDF framework to identify CSTIPs’ needs for PrEP service provision. In the Development Phase, I generated an item pool for the anonymous, self-administered online questionnaire in Traditional Chinese (the official written language of Taiwan) using the findings in the Formative Phase. All preliminary items were then evaluated by eight CSTIPs through cognitive interviews and by six experts in my survey committee. Based on the feedback from both sources, I optimised the content, logic flow and layout of the final questionnaire in Traditional Chinese. Results: Formative Phase: Unmet PrEP prescribing needs of CSTIPs are identified in all three COM-B components and 14 TDF domains. CSTIPs expressed a wide range of needs for delivering PrEP services in the National PrEP Programme due to limited capabilities, missed opportunities and low motivation. Their needs highlighted the political and social milieux of Taiwan’s centralised PrEP care system and multi-faceted PrEP stigma. Types and the extent of unmet needs varied between PrEP adopters and non-adopters, but both groups reported a marked demotivation, including: lack of professional confidence; personal biases and moral judgement towards MSM; disbelief in positive health outcome of PrEP; minimal incentives and anticipated administrative burden of and distrust in governmental programmes. Development Phase: Of all 76 initial items, 15 remained unchanged and six were removed after three rounds of cognitive interviewing with revisions. Comprising 70 items in four sections, the final Traditional Chinese questionnaire is culturally sensitive, theoretically attested and primed for measuring the needs of CSTIPs. Items on all 14 TDF domains were designed to measure CSTIPs’ needs for delivering PrEP services. Except for the Emotion domain, all TDF domains were covered by items identifying stakeholders who could satisfy CSTIPs’ needs. Conclusion: The wide-ranging needs of CSTIPs for PrEP prescribing are fully captured by the COM-B/TDF framework. It is feasible to develop an acceptable comprehensible survey to measure CSTIPs’ unmet needs that can guide professional development interventions. Future administration of the developed questionnaire will inform actionable and needs-based policies that decentralise PrEP service delivery, which may increase users’ accessibility to PrEP services, ultimately maximising the impact of PrEP on ending AIDS in Taiwan.