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One of the most surprising consequences of quantum mechanics is the entanglement of two or more distant particles. Although questions regarding fundamental issues of quantum theory still exist, quantum entanglement has started to play important roles in practical engineering applications. Quantum imaging is one of these exciting areas. Quantum imaging has demonstrated two peculiar features: 1) reproducing "ghost" images in a "nonlocal" manner and 2) enhancing the spatial resolution of imaging beyond the diffraction limit. In this paper, we start with the review of classical imaging to establish the basic concepts and formalisms of imaging. We then analyze two-photon imaging with particular emphasis on the physics of spatial resolution enhancement and the "ghost" imaging phenomenon.