Nonlinear optics, which governs the interaction of light with various media, offers a whole raft of useful applications in photonics, including multiphoton microscopy and multiphoton lithography. It also provides the physicist with a remarkable range of opportunities for generating light with interesting, novel, and potentially useful properties. As a particular example, entangled-photon beams generated via spontaneous optical parametric down-conversion exhibit unique quantum-correlation features and coherence properties that are of interest in a number of contexts, including imaging. Photons are emitted in pairs in an entangled quantum state, forming twin beams. Such light has found use, for example, in quantum optical coherence tomography, a quantum imaging technique that permits an object to be examined in section. Quantum entanglement endows this approach with a remarkable property: it is insensitive to the even-order dispersion inherent in the object, thereby increasing the resolution and section depth that can be attained. We discuss the advantages and disadvantages of a number of techniques in multiphoton and entangled-photon imaging and lithography.