Coherence, defined by the observability of interference, is fundamentally related to basic optical concepts such as the wavefront, point source, and extended object. Coherence theory provides the correct physical description of light emitted by polychromatic, extended objects. In practical terms, it defines the conditions under which such complex natural sources can be treated as if they were simple, point-like, monochromatic sources. Understanding coherence is critical for several reasons: \(\bullet \) The temporal coherence of light imposes fundamental limitations on the design of interferometers for metrology. \(\bullet \) The spatial coherence of light is defined by the angular size of an incoherent reference source. \(\bullet \) The spectral properties, noise, and resolution of images are dependent on the coherence of the illumination. In general, imaging with coherent light provides lower resolution and higher noise due to speckle and interference artifacts. \(\bullet \) Coherent light is used in holografy for engineering of coherent optical wavefronts.