Principles of Statistical Radiophysics is a four-volume series that introduces the newcomer to the theory of random functions. It aims at providing the background necessary to understand papers and monographs on the subject and to carry out independant research in fields where fluctuations are of importance, e.g. radiophysics, optics, astronomy, and acoustics. Volume 2, Correlation Theory of Random Processes, presents the correlation theory of nonstationary processes paying particular attention to periodically nonstationary processes. Physical phenomena like interference, coherence and polarisation of random oscillations, thermal noise in discrete dynamical systems, and the spectral representations of random actions on discrete systems are dealt with.