Serious mental illness (SMI; schizophrenia-spectrum and bipolar disorders) affects Veterans at high rates and is associated with major difficulties in psychosocial and physical function. The burden of SMI worsens over the lifespan, which makes older Veterans with SMI particularly at risk for functional decline. Existing behavioral treatments are effective at improving psychosocial function but do not increase physical function in older Veterans with SMI. The lack of targeted interventions for physical function in older Veterans with SMI is troubling given that compromised physical function is related to more falls, hospitalizations, nursing home admissions, as well as early mortality. Older Veterans with SMI have significantly lower physical function across mobility, endurance, and strength domains compared to both age- and sex-based population norms and to older Veterans without SMI, further highlighting the magnitude of functional decline. Exercise is an established intervention for increasing all domains of physical function (i.e., mobility, endurance, strength) in older Veterans. Despite a growing research base on exercise for adults with SMI, exercise trials for adults with SMI have been limited by largely focusing on endurance, excluding older persons, and having high dropout rates. Environmental difficulties (e.g., lack of transportation), low motivation, and medical issues affecting older Veterans with SMI contribute to low engagement rates and high dropout rates in facility-based exercise programs. Individualized home-based exercise programs, which are safe and effective for older Veterans with health challenges, could address the main barriers to exercise in older Veterans with SMI by promoting greater accessibility and individual tailoring. To this end, this Career Development Award (CDA-2) will take a systematic approach to develop, refine, and feasibility test a home-based exercise program aimed at improving function in older Veterans with SMI. The home-based exercise program will be modeled after VHA’s Home- based Cardiac Rehabilitation Program, a 12-week program comprised of individualized home-based exercise prescription and weekly telephone health coaching. Further, motivational strategies for exercise (e.g., goal- setting, tracking progress, problem-solving) will be integrated into Home-based Cardiac Rehabilitation telephone health coaching calls. Guided by the successive cohort design, an initial exercise program manual will be developed based on Home-based Cardiac Rehabilitation (with motivational strategies) and qualitative input from key stakeholders (older Veterans with SMI, clinical staff and leadership serving older Veterans with SMI). The initial manual will be tested in a small 12-week open trial of two successive cohorts of older Veterans with SMI (n=5 in each). Qualitative and quantitative feedback on the program and research procedures will be collected and analyzed after each cohort and used to refine the program ma...