The 3D microarchitecture of the myocardium underlies the mechanical and electrical properties of the heart. Cardiac Diffusion provides a non-invasive means to understand not only the dynamic changes in healthy myocardial microstructure during cardiac contraction but also the pathophysiological changes in the presence of disease. This technology offers tremendous potential to enable improved clinical diagnosis through novel microstructural and functional assessment. This talk will introduce the 3D myocardial microstructure, review in vivo cardiac diffusion methods, and clinical applications to date.