Cognitive decline in older adults is common, with 1400 new cases of dementia diagnosed in Australia each week (Access Economics, 2009). Clinical groups at risk for cognitive decline are older adults with subjective member complaints (SMC) or Mild Cognitive Impairment (MCI). SMC are defined by concerns about deteriorating cognitive function without objective evidence of impairment on cognitive testing, whereas MCI requires both subjective concern and objective cognitive impairment. Observational studies suggest physical activity (PA) is associated with positive health outcomes, and a small number of randomised controlled trials (RCTs) have shown improvements in cognition following PA interventions (Lautenschlager et al. 2008). However, the mechanisms underlying positive cognitive effects of PA are not yet fully understood. Further, while national recommendations for PA for older Australians suggest 30 minutes of moderate-intensity aerobic activity, five days a week, current estimates suggest that 83% of Australian’s aged 75 years and older do not achieve this recommended amount (ABS, 2011). This presentation overviews methodology and findings from two current PA RCTs in older adults at risk of cognitive decline; the AIBL active study and the INDIGO trial. AIBL active explores possible mechanisms underlying positive cognitive effects of PA. Specifically, we have recruited a cohort 108 of SMC and MCI individuals who have at least one risk factor for cardiovascular disease (CVD), to examine whether participants with SMC and MCI who are randomised to a 24-months PA program demonstrate less progression of CVD measured on MRI, less cognitive decline and depression, and better quality of life at 24 months than participants randomised to usual care. INDIGO is a novel trial exploring adherence to PA interventions, specifically examining whether a home-based 6-month PA intervention with individual goal-setting and volunteer mentors can significantly increase PA levels in sedentary older adults at increased risk of developing Alzheimer’s Disease.