Each participant will undergo the same battery of balance and gait tests. Although STN-DBS settings are typically stable at 12 months post-surgery, participants will be allowed to follow up with their neurologists as needed for programming and medication adjustments. All changes in programming settings and medication dosages will be noted. A rater, blinded to group assignment, will collect all outcomes at each time point. Participants will be tested in the following conditions: 1) OFF stimulation and OFF medication and 2) ON stimulation and ON medication. Testing participants OFF stimulation/OFF medication will allow us to determine if the addition of PT after STN-DBS affects balance and gait independent of other treatments. OFF medication is defined as greater than or equal to 12 hours since the last intake of anti-PD medication. For OFF stimulation/OFF medication testing, participants will arrive to the laboratory OFF medication but with stimulators on. The stimulators will be turned OFF upon arrival at the laboratory and testing will commence 45 minutes after the stimulators are turned OFF. Stimulators will be turned back on upon completion of this testing session. The investigators anticipate the total time for this laboratory visit to be 2 hours. Testing ON stimulation/ON medication will provide insight into how participants perform on an everyday basis. ON medication is defined as 1-1.5 hours after medication intake. For ON stimulation/ON medication assessments, participants will arrive to the laboratory 1-1.5 hours after taking their normal anti-PD medication dose with stimulators on and stimulators will remain on throughout the session. Assessments will occur at baseline and at 8 (i.e. post-test) and 12 weeks (i.e. follow up). Assessments will take place on two separate days and the order of testing condition (OFF medication/OFF stimulation vs. ON medication/ON stimulation) will be randomized. The Movement Disorders Society-Unified Parkinson Disease Rating Scale III (MDS-UPDRS III) will be administered in each condition.