Introduction: With aging and the onset of neurodegenerative diseases such as Parkinson's disease, cognitive and motor disorders can appear and have repercussions on the activities of daily living. Walking disorders and consequently the risk of falling, in normal and pathological aging, represent a major public health issue. Among the factors that can explain these disorders, prefrontal activity during walking in aging and Parkinson's disease has been relatively little investigated. The objectives of this work were to: 1) better understand the changes in prefrontal activity during walking with advancing age and in the presence of Parkinson's disease and 2) determine to which extent an intensive physical rehabilitation program could modify prefrontal activity in these patients. Methods: To address the first objective, 93 participants divided into 4 groups (young adults, youngest-old adults, older adults, and Parkinson's disease patients) were included. Prefrontal activity (changes in HbO2) was measured using functional Near Infrared Spectroscopy (fNIRS) during different walking tasks varying in complexity. To address the second objective, prefrontal activity during walking was measured before and after an intensive exercise-based rehabilitation program in patients with the same device. Test-retest reliability of prefrontal activity during usual walking was also examined in order to ensure that changes are actually due to the rehabilitation program. Results: The results show a progressive increase in prefrontal activity with age and Parkinson's disease. This increase is present as early as 55 years of age in single-task walking condition, without alteration of walking performance compared to young adults. This may reflect a greater demand on executive resources to perform the walking task. However, in the dual-task condition, older individuals and patients show prefrontal overactivation, associated with poorer walking performance. This suggests that older adults and patients are no longer able to effectively compensate for age-related deficits to perform the dual-task walking. After an intensive rehabilitation program, prefrontal activity during single-task walking decreased in Parkinson's patients, indicating a decrease in the demand for executive resources and thus a gain in the automaticity of walking performance. Moderate, almost excellent between-session test-retest reliability for changes in HbO2 was found meaning that fNIRS may be used as a reliable tool for rehabilitation studies in patients with PD. Conclusion: In conclusion, this work allows us to better understand the neurophysiological modifications with advancing age and Parkinson's disease. From a clinical point of view, this work could also help to better identify people at risk of falling.