Twentieth-century hypotheses attributing a substantive role to aging in Parkinson disease (PD) pathogenesis have been countered by evidence from clinical, pathological, and biochemical investigations. However, age influences the clinical progression of PD. Several studies have demonstrated that advancing age is associated with a faster rate of motor progression, decreased levodopa responsiveness, more severe gait and postural impairment, and more severe cognitive impairment and the development of dementia in patients with PD. A model for the relationship between PD and aging is proposed that incorporates the following 3 elements: (1) There occurs a superposition of a topographic gradient of neuronal loss in brainstem and basal forebrain structures related to the disease process and an aging-related temporal gradient. (2) While PD is a chronic progressive disorder, the most important determinant of clinical progression is advancing age rather than disease duration. (3) The effects of the disease process and aging on nondopaminergic structures involve a biologic interaction. The model implies that understanding the degenerative process in nondopaminergic structures in PD as it relates to molecular mechanisms accompanying the aging of the nervous system may create opportunities for interventions affecting the clinical progression of the disease.