Ghost imaging (also called "coincidence imaging", "two-photon imaging" or "correlated-photon imaging") is a technique that produces an image of an object by combining information from two light detectors: a conventional, multi- pixel detector that does not view the object, and a single-pixel (bucket) detector that does view the object. Two techniques have been demonstrated. A quantum method uses a source of pairs of entangled photons, each pair shared between the two detectors, while a classical method uses a pair of correlated coherent beams without exploiting entanglement. Both approaches may be understood within the framework of a single theory.