In Chap. 1 , classical optical interferometry was discussed. In this chapter, we look at interference effects more closely. In particular, we wish to consider cases where the interference is inherently quantum, in the sense that it cannot be mimicked by classical optical fields obeying Maxwell’s equations. Essentially what we will find is that classical interference arises from single-photon interference:Interferencesingle-photon each photon interferes only with itself. For example, in the Young two-slit experiment, each photon has an amplitude to go through one slit and an amplitude to pass through the other. The interference arises because those two amplitudes interfere with each other for each photon individually. To have intrinsically quantum interference, it will be necessary to have two-photon interferenceInterferencetwo-photon (or more generally, multi-photon interference): a two-photon amplitude for a pair of photons will interfere with another amplitude for the same two-photon pair to take a different path.