This study will evaluate gait when individuals with Parkinson's disease (PD) are walking while talking to see 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.