This thesis investigates how magnetic resonance imaging can improve planning and evaluation of proton beam therapy for uveal melanoma, showing that high-resolution ocular MRI provides geometrically accurate images for treatment planning and enables reliable tumor measurements, with improved accuracy over ultrasound in anterior tumors. The work demonstrates that commonly used tumor models overestimate tumor size and volume, whereas MRI-based three-dimensional delineations provide more precise patient-specific characterization. Quantitative and clinical MRI approaches, including perfusion- and diffusion-weighted imaging, are evaluated for response monitoring and indicate that simpler clinically applicable methods are sufficiently reproducible. The thesis proposes an MRI-based workflow that can eliminate invasive marker placement, with potential gains in treatment accuracy and efficiency.