Pre-exposure prophylaxis (PrEP) is highly-effective in preventing new HIV infections among gay, bisexual and other men who have sex with men (GBMSM) and transgender women (TW), but relies on engaging these high- risk communities at multiple steps along the PrEP continuum of care. Studies show that a minority of PrEP- eligible GBMSM are taking PrEP, and TW are less likely than GBMSM to have heard of PrEP, talked with their provider about PrEP, used PrEP in the past 6 months, and been adherent to PrEP. An existing HIV testing and PrEP program led by members of our study team in San Diego, CA found that over 60% of GBMSM and TW participants eligible for PrEP were not interested in starting it; of those who did start PrEP, only 5.8% remained on PrEP after 3 months. Clearly, innovative interventions that optimize the PrEP continuum of care for diverse GBMSM and TW are needed. We propose here a theoretically-grounded (in the IMB model), effective, accessible and sustainable mHealth PrEP care continuum intervention for GBMSM and TW, called eSTEP (Electronic Support To Engage with PrEP). The eSTEP WebApp will be tailored by sexuality and gender status and include a set of interactive components delivered before in-person HIV testing (e.g., PrEP information on the benefits of PrEP) and after the participant tests HIV-negative and agrees to start PrEP (e.g., adherence self-monitoring). Aim 1: Use an iterative development process to gain feedback on eSTEP content and features through focus groups with GBMSM and TW and a community advisory board (CAB). 6 focus groups with 18-45 year-old GBMSM and TW (6 participants/group; > 50% Black/Latinx) will be conducted at 3 key points in the intervention development cycle to create the tailored (separately for GBMSM & TW) IMB-related content and features of eSTEP. A CAB of GBMSM (n=3) and TW (n=3) will be established to provide guidance between rounds of focus groups. Aim 1 will culminate in usability testing among 3 GBMSM and 3 TW to finalize eSTEP. Aim 2: Conduct a pilot randomized controlled trial of eSTEP to assess its feasibility, acceptability, and preliminary impact among GBMSM and TW in preparation for a large-scale R01. 120 (50% racial/ethnic minority) GBMSM (n=85) and TW (n=35) who report risk factors that qualify for PrEP eligibility will be randomized (2:1) to the eSTEP intervention (n=80) or a usual HIV testing control (n=40). The eSTEP group will have immediate access to pre-HIV testing components. All study participants will be offered free in-person HIV testing and, if eligible, PrEP at a university-based HIV prevention and treatment clinic. eSTEP participants who begin PrEP will unlock PrEP adherence and persistence components. Follow-up study visits will be conducted at 3- and 6-months to collect biological, clinical, and self-reported outcomes to assess eSTEP’s feasibility, acceptability, and preliminary outcomes (HIV test completion, PrEP initiation, PrEP adherence, and PrEP appointment attendance)...