As presaged in Chap. 1, we will primarily be interested in understanding entanglement in holographic field theories. But before we get to this particular set of quantum systems, it is useful to build some intuition in a more familiar setting. In this and the next section, we will therefore focus our attention on getting some insight into the concept of entanglement and learn some of the techniques which are used to characterize it. The discussion here will also serve to build some technical machinery which will be useful in the holographic context.