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The development of a neuroprotective or disease-modifying therapy is the major unmet need in the management of Parkinson's Disease (PD) and the goal of much clinical and scientific research. However, despite enormous efforts and expense, no disease-modifying therapy for PD has been approved to date. Historically attempts to define such a therapy have been limited by confounding symptomatic/pharmacologic effects of the study intervention and the lack of a clear and well-defined regulatory and clinical development pathway that leads to a disease-modifying indication. Further, the costs of the development program average 1 billion dollars with a duration of 10 to 13 years. As a consequence, many pharmaceutical companies are reluctant to test novel therapies despite the recent scientific advances and promising candidate targets and approaches. In the present review we describe previous studies aimed at defining a disease-modifying drug and discuss their limitations. We also consider some of the modern approaches and trial design for drug development that will hopefully pave the way toward identifying and gaining regulatory approval for a disease-modifying therapy in a relatively efficient and cost-effective manner.