The primary goal of this study is to test whether 12 weeks of high-intensity aerobic exercise can treat fatigue in Parkinsons disease (PD). The study will be a randomized multi-site controlled trial with follow up. Clinically fatigued persons with PD will be allocated to either 12 weeks of high-intensity aerobic exercise or to a waitlist control group receiving high-intensity resistance exercise after 24 weeks of habitual lifestyle (control period). It is hypothesized that persons with PD receiving 12 weeks of high-intensity aerobic exercise will show superior effects on perceived fatigue (i.e., clinical relevant reductions) when compared to the PD control group (primary hypothesis), and that these effects are sustained after 12-weeks of follow up (secondary hypothesis).