This talk will cover the physical origins of T1 relaxation, present biophysical models for it in tissue, and explain why it is an important biomarker for many pathologies. It will then present some common techniques and sequences for measuring it, the most common of which are saturation and inversion recovery. Finally, it will present some of how obtaining T1 maps can be used both clinically and, to a lesser extent, as a tool in more basic research.