Imaging of response to oncology treatments, either on clinical protocol or as part of standard practice, is a complicated process that has evolved during the last 10 years due to the improvement of existing imaging technologies and the introduction of newer modalities. Diffusion magnetic resonance imaging is a technique that measures the mobility of water within tissues and, as such, may function as a surrogate marker for both tissue cellularity and response to treatment that occur earlier than usual measures of tumor response. This review highlights the development of this technique and the state of current clinical understanding of its utility.