In recent decades, entanglement has gone from being viewed as a quantum oddity to being seen as a quantifiable resource that be exploited to facilitate applications. In this chapter, entanglement is examined a little more closely, and a range of approaches to detecting and quantifying it are discussed. In particular, Bell states and Schmidt decompositions are defined in order to formalize the description of entangled states, and then measures of entanglement are defined. These measures include concurrence, quantum discord, fidelity, and trace distance, in addition to tests of entanglement such as entanglement witnesses and the PPT criterion.