The book deals with correlated photons in quantum optics, the methods of producing them and a number of applications in biphoton spectroscopy and polarisation, graphics of the biphoton field. In the majority of the experiments described in the book, the correlated pairs of the photons (by photons) were produced in the process of spontaneous parametric scattering of light. Such a pair of photons is the unit quantum object in the so-called entangled state. This state is described by a single wave function and has a number of unique statistical properties. The correlated photons in the pair are rigidly connected together by the area and moment of nucleation, frequency and the direction of dispersion. The intensity of the flux of the biphotons is directly associated with the brightness of zero fluctuations of electromagnetic vacuum.