For patients with Parkinson’s disease (PD), exercise affords many health benefits. Recently, we performed the Park-in-Shape study where 6 months of high-intensity exercise on a stationary bicycle at home was compared to a stretching control group. We provided level 1 evidence that aerobic exercise offers marked symptomatic relief of PD. Adherence was excellent, but depended on remote supervision. Without direct coaching, levels of physical activity remain stubbornly tough to modify. Also, not all patients can or want to use a stationary bicycle. Recent advances in mobile technology offer a new strategy to overcome these challenges and to scale exercise interventions at an unprecedented level. Previous work recommends that mobile technology should use various engagement strategies in order to improve compliance and to actually increase physical activity. Here, we take an innovative step by studying a carefully designed, fully decentralized, smartphone-based, titrated program to increase long-term exercise in daily life. We will equip the patients’ own smartphones with a customized app with state-of-the-art engagement strategies to motivate patients to increase their physical activity. We will perform a large-scale, double blind, randomized clinical trial, randomizing PD patients (n=450) to achieve different levels of increased step counts: 10-50-100 or 200% (10% being the control group). The same smartphone will also monitor outcomes (built-in sensors for step counts, plus a second app to measure PD symptoms and signs, as well as VO2Max remotely). We aim to: (1) study whether our smartphone app can increase physical activity in PD patients for long periods of time (12 months); (2) test the effectiveness of physical activity on attenuating PD symptoms and signs, and also to improve aerobic capacity (as a potential intermediate to influence PD pathophysiology); (3) study the dose-response: what increases (in step count) are needed to achieve such clinical benefits; and (4) study the validity of a remotely performed VO2Max test in PD patients as compared to the gold standard lab test. We expect this approach to be highly scalable, allowing for future large-scale studies to examine the long-term symptomatic effects of exercise, also in medically underserved areas, and also for chronic conditions other than PD. Additionally, our work opens new avenues to test if exercise could have disease-modifying properties in PD.