Parkinson's disease (PD) is a common neurodegenerative disorder characterized by relentlessly progressive motor and nonmotor clinical features. In this paper, we offer a comprehensive overview of progression in PD, covering the heterogeneous symptomatology crucial for monitoring progression from clinical, pathological, and biomarker perspectives. We also discuss prevailing theories concerning the underlying pathobiology driving progression in PD and summarize the literature on emerging biomarkers that are expected to facilitate early prognosis and effective monitoring of disease progression.