This research explores materialisations of treatment promise among young adults living-with HIV, and on treatment, in London, UK. The longitudinal qualitative study draws from data generated across repeat interviews with young adult participants, aged between 18 and 29 years, recruited from two inner London clinics. Guided by feminist science theory and science and technology studies, the analysis explores how treatment promise unfolds, is negotiated and experienced, in process with a situated living. Biomedical makings of promise, though affecting materialising realities, are made complicated in the immediate vitality of young-adult lives. Intervention effect is a situated accomplishment (Stengers 2005) that while making-safe is at once open and vulnerable to change, troubling assumed therapeutic securities. HIV realities and therapeutic potential are made and remade as virus, human and biomedical bodies come to hold together, and transform, in new ways (Barad 2007; Race 2014). Governing processes, while not determining, fold in and affect how biomedical promise is collectively made to matter and put to use (Barad 2007; Rhodes & Lancaster 2019a), and how affordances of treatment come to be made socially, politically and culturally relevant. Yet efforts to stabilise particular makings of promise are at once troubled, with interferences enacted in moments of quiet activism – the tentative doing-differently – of young adults’ lived worlds and relationships. Negotiations of therapeutic possibility are likewise affected by the particularity of the young-adult experience at this epidemic juncture. As past and future enfold in to the present-moment, an assumed linearity of time is disrupted (Barad 2017). Living in the ‘now’, while governed and negotiated in relation to envisioned futures (Lancaster & Rhodes 2020), is at once in process with the past – the still doing of HIV histories and memory (Barad 2017). The thesis concludes with a discussion of the risk of biomedical accomplishment eclipsing the still complex social, political and cultural negotiations at work in practices-of-living in the post-treatment era, and the imaginary commitment necessary to conceive and hold open other possible makings of HIV realities and futures.