MR imaging is widely used in the radiological diagnosis of oncology patients as it provides excellent soft tissue differentiation using routine anatomical MR imaging. A variety of MR acquisition sequences are available which can yield images of biophysical, physiological, metabolic, or functional properties of tissues. Imaging of response to oncological treatments has traditionally used single or multidirectional measurements of tumour dimensions following completion of therapy. Development of an MR imaging biomarker that would allow for early prediction of tumour response to therapeutic intervention would be a significant achievement as it could individualize clinical management of cancer patients in a timely fashion and improve outcome. This goal is very important as standard risk factors currently used in patient assessment cannot account for the variable and unpredictable treatment responses encountered by patients with similar risk profiles. This chapter will overview the use of diffusion-weighted MR imaging (DW-MRI) as a method of providing a potentially early surrogate marker of response to therapy in oncological imaging.