To determine the Responsiveness of the iMOBILITY in response to intensive physical therapy exercise programs. (Is it sensitive to change?) Although exercise is thought to be the most effective intervention for balance and gait in PD (compared to dopaminergic medication or DBS surgery), the best exercise program for mobility in PD is unknown. The iMOBILITY will be used to quantify balance and gait performance before and after two PT-supervised, intensive, exercise programs, expected to improve balance and gait. The first program is a published Treadmill training program and the second is the investigators new Agility training program with sensorimotor progressions, targeted at specific impairments that underlie the abnormalities of balance and gait in PD (developed for the Kinetics Foundation). This pilot clinical trial will randomize 40 PD subjects into the two exercise programs at OHSU in preparation for a larger clinical trial to determine the most effective exercise for mobility disability in PD. The effects of exercise will be compared with no treatment during a 5-week delay prior to start of exercise.