Diffusion MR imaging techniques are based on the molecular mobility of water, which is sensitive to interaction with intracellular elements, macromolecules, cell membranes, the density of cells, and microstructural organization. Disease processes that alter cell-water homeostasis, cell density, and cytoarchitecture affect water mobility and are quantifiable by diffusion MR imaging methodologies. Similarly, therapeutic intervention that alters these properties may be monitored for efficacy via diffusion MR imaging. This article outlines basic technical concepts and applications of diffusion imaging with an emphasis on oncology.