This chapter provides a brief introduction to charged particle optics. It is not meant to be a survey; rather, our intent is to provide enough information to uynderstand how the optical system of focused ion beam system works. We also provide a method to define the resolution of a focused ion beam system in a way that gives a metric for optimization or for comparison of different systems. The resolution definition is based on a method devised for the wave optical treatment of electron beam systems, and so a brief outline of wave optical methods is also provided.