This chapter explains quantization rules for Maxwell's macroscopic equations for a field in an unbounded medium, the optical nonlinearity of matter being neglected. It deals with photon statistics in various states of the field and its connection with pulse statistics at the output of the photomultiplier tube. Among the various types of instantaneous states of the quantum oscillator, coherent states stand out as being, in a sense, closest to the state of a classical oscillator with a definite coordinate and velocity. The chapter looks at possible quantum states of the field and their change in time in the presence of given sources of the field. It examines correlation functions symmetry; the relationships of the various correlation functions with one another, with the microparameters of systems, and with their green functions. It should be emphasized, however, that a photon in a medium is a quasiparticle with a complex dispersion law and a finite lifetime.