This PhD thesis describes a series of studies on the pre-clinical development and application of magnetic resonance imaging (MRI) techniques for evaluation and optimization of photodynamic therapy (PDT). PDT is an emerging cancer treatment, based on a light-activated drug, called the “photosensitizer”. the photosensitizer is typically administrered intravenously houts to days before treatment to facilitate uptake of the photosentitizer in the tumor with an optimal tumor to background ratio. The treatment phase consists of light irradiation of the tumor, leading to generation of oxygen radicals by the photosensitizer. These highly reactive oxygen radicals induce damage to proteins, cell membranes, and other vital targets. The cancer treatment effect either occurs through direct tumor cell death or through vascular shutdown, depending on the accumulation characteristics of the photosensitizer in the tumor microinvironment.