<p>Quantum imaging encompasses many techniques, from the use of cameras that detect single photons with picosecond temporal resolution, to the use of nonlinear materials to create pairs of photons correlated over many different degrees of freedom, or even utilizing the interference between two of these photon-pair sources. This article focuses on the various imaging techniques using correlations between photon pairs, reviews the basic theory required to understand the method, discusses the practicalities of implementing these approaches, presents ways in which such systems might surpass classical limits and considers the applications that these advances might enable. Finally, we discuss future directions of the field of quantum imaging and the challenges that remain, both in terms of required new technologies and areas of physics in which our understanding may be incomplete.</p>