This study will evaluate gait when individuals with Parkinson's disease (PD) are walking and how walking changes when challenged to perform a functional cognitive task simultaneously. By looking at walking alone and walking with varying cognitive loads the investigators will be able to determine motor and cognitive factors vulnerable to interference in PD. Each is important to understand so that training can address components of walking that become impaired when someone is distracted by a common cognitive task and so that the intensity of treatment matches the level of task difficulty. Using this initial data, the investigators will establish a protocol to improve walking taking into account the unique features of PD, including bradykinesia, freezing of gait, stiffness, and problems with memory and attention. The investigators will evaluate the potential for this treatment to improve walking and improve or maintain cognitive abilities necessary to multitask.

In this study participants will be asked to participate in a training study to examine whether a new walking intervention that combines cognitive training with walking on a treadmill improves walking more than gait training or cognitive training alone. Participants with Parkinson's disease (PD) will come to the lab for a complete gait assessment using a motion analysis system that will record leg motion and muscle activity during comfortable walking. During the initial evaluation, participants will also complete cognitive assessments and surveys about community activities and quality of life related to PD. Participants will be divided into two training groups and asked to come to the lab three times a week for four weeks during the intervention. One week and one month after training, participants will return to the lab to complete all the same assessments that were performed during the initial visit. The results of the three testing sessions (initial, one week and one month) will be compared between and within groups to see if any of the interventions provided unique benefits to the health of persons with PD.

Inclusion Criteria:

• Clinical diagnosis of Parkinson's disease with our without cognitive impairment
• ambulatory for at least 200 feet without an assistive device (cane, walker, etc)

Exclusion Criteria:

• recent orthopedic injury (e.g., fracture, sprain, muscle pull, herniated disc)
• concurrent neurological illnesses including stroke, brain cancer, spinal cord injury, and diabetic neuropathies.
• cardiac insufficiency limiting ability to participate in walking study

Assessments are performed by a clinician blinded to participant group (training vs. waitlist control) allocation.