The most commonly used MRI sequences, T1-weighted, T2-weighted, and T2*-weighted are used to generate images whose contrast depends on T1, T2, and T2* relaxation values. However, the images themselves are not direct measures of T1, T2, and T2* which are physical characteristics of the tissues. T1-weighted, T2-weighted, and T2*-weighted images are used to identify areas of abnormally bright or dark signals, but they do not tell us anything specific about changes in tissue. In this presentation you will learn how relaxation arises, how it can be measured, and what it can tell us about the brain’s microstructure.