Project Background/Rationale. Progressive cognitive impairment leading to dementia is an important component of Parkinson's disease (PD), contributing to significant levels of disability. The number of veterans who will develop PD and, in turn, the number of veterans with PD who develop dementia is likely to increase substantially. Given the profound negative health and social consequences associated with the development of dementia, it is critical to identify interventions that effectively slow the decline of cognitive function to prolong the time to onset f dementia. Based on the results of prior studies, physical activity is one of the few nonpharmacological interventions that holds promise in slowing cognitive decline. Project Objectives. We propose to build on our experience with objective, physiologically-based measurements of physical activity in randomized, controlled trials (RCTs), to conduct an RCT of a home-based physical activity intervention program among patients with mild cognitive impairment in PD (PD-MCI) - a group at high risk of experiencing further cognitive decline and most likely to benefit from physical exercise. We hypothesize that patients with PD-MCI in the intervention arm will have less cognitive decline compared with those in the control arm. Project Methods. Community-dwelling veterans with PD-MCI will be randomized either to a smartphone-based physical activity intervention to promote brisk walking or a health education control intervention. We will test the effects of the walking intervention over 6 months for the primary outcome of Alzheimer Disease Assessment Scale-Cognitive Subscale and for the three secondary outcomes of Stroop Test, Verbal Fluency Test, and California Verbal Learning Test. Furthermore, we will test for the durability of these effects with a tapered physical activity dosage through 18 months of follow-up. Information derived from this study could be easily incorporated into clinical practice guidelines and disseminated widely at relatively low cost withi the VA Healthcare system and beyond.