Optical Pumping of atoms involves manipulating atomic populations among the different magnetic quantum number sublevels. The most common technique for changing the magnetic sublevel distribution of atoms is by the fundamental interaction of atoms with light. This book will describe this fundamental interaction in detail. It includes a review of experimental details for the preparation of optically-pumped systems. Up-to-date applications of this phenomenon are illustrated, e.g. imaging in humans based on optically-pumped atomic systems, laser cooling, and trapping of atoms, technologies of magnetic resonance imaging.