This chapter purposes to elucidate some of the fundamental relationships involving fluctuation phenomena. For a system in thermodynamic equilibrium, it is a well-known fact that fluctuations ordinarily remain small in magnitude as compared with mean values. When one of the instabilities sets in, however, the fluctuations are expected to grow enormously; the system goes over to a turbulent state. The fluctuations are usually represented by their spectral functions, which are in some cases called form factors. The spectral functions of fluctuations are connected to the linear response functions of the system. Various spectral tensors of the field fluctuations in the plasma may be calculated through the superposition technique of the dressed test particles. The microscopic electric field in the plasma fluctuates as the charged particles move randomly. The spectral tensor of fluctuations can be calculated through a combination of the macroscopic laws of electrodynamics and the superposition principle of the dressed particles.