Structural, diffusion, functional, and spectroscopic magnetic resonance imaging (MRI) each provide meaningful information about brain changes that accompany Parkinson's disease (PD) progression. These MRI methods hold promise to elucidate the benefits and complications of neuroprotective and restorative therapeutic intervention for PD. This chapter describes the general role of imaging in clinical trials and defines the characteristic of an ideal outcome measure. The ideal outcome marker should fulfill the following criteria: first, the relationship between the outcome marker and the desired clinical effect should have been clearly established. Second, the outcome marker should be objective and reproducible, allowing assessments of treatment efficacy with high statistical confidence. Finally, the assessment should be non-invasive and well tolerated. Since MRI meets some of these criteria, in particular objectivity and noninvasiveness, the ability of MRI to benefit assessments of clinical trials is increasingly being investigated.