This paper describes basic physics as well as clinical applications of diffusion-weighted magnetic resonance imaging. This is a technique that provides complementary information to conventional imaging sequences and it is applied in the field of oncologic imaging. This paper focuses on its specific application in head and neck, mainly in cancer patients, for characterization of primary tumors, and also for monitoring and predicting treatment response after chemotherapy or radiation therapy. Last, although diffusion-weighted imaging is shown to add value in several areas by being part of the multi-parametric magnetic resonance imaging approach, there are some unsolved challenges, which are proposed as future work.