Increasing numbers of persons living with HIV (PLWH) are living to older ages. Older PLWH are at increased risk for a wide spectrum of co-morbid conditions such as cardiovascular disease, frailty phenotype, and mental health disorders. Practical primary care-based interventions are needed for older PLWH that target lifestyle factors to reduce complications of aging and improve healthspan. Most PLWH engage in less physical activity (PA) than is recommended by public health guidelines. Thus, increasing PA can lead to numerous mental and physical health benefits among PLWH. Current studies in PLWH typically focus on the impact of supervised exercise. However, there are many barriers to this type of PA. Lifestyle physical activity (LPA) programs that focus on integrating PA, especially walking, into everyday life, may be more successful in producing a sustained increase in PA. The long-term goal of this research is to develop an effective and efficient primary care intervention that increases LPA among older PLWH. We leverage the Multiphase Optimization STrategy (MOST) as a framework for developing, optimizing, and evaluating a multicomponent behavioral intervention. MOST is comprised of three phases. First, in the preparation phase, an empirical and theory-driven conceptual model is developed that identifies differentiable intervention components and specifies optimization criteria (i.e., goal of intervention development). We have completed this phase. Second, in the optimization phase, the impact of each intervention component on the desired outcome is empirically examined. The goal is to determine which components to include in a final intervention package that is optimized for efficacy and efficiency. The proposed study reflects the second phase, or an optimization trial. Finally, in the evaluation phase, the optimized intervention package is evaluated for efficacy in a traditional RCT; this will be the next step in this line of research following the currently proposed project. In this project, we will test the impact of three separable intervention components on steps per day after 4 months of intervention. We will recruit 208 PLWH of age ≥50 year classified as low-active. All participants receive component '0': brief advice to exercise and a Fitbit activity tracker. We then test 3 additional components in a factorial design: 1) PA Coaching-focused on goal-setting and self-monitoring; 2) Cognitive-Behavioral intervention for PA (CB-PA)-focused on identifying values and addressing internal barriers to PA such as pain, depression, or fatigue; 3) Social Support for PA (SS-PA)-a component which leverages the social functionality of the Fitbit app, promoting social support and friendly competition. The primary outcome will be steps per day during Month 4. The observed main and interaction effects for the 3 candidate components will guide selection of components to be included in an optimized intervention that will be tested in a future RCT. We will also examine potential mediators for each intervention component, as well as secondary outcomes including perceived physical health, mental health, cardiovascular disease risk, and the frailty phenotype.