Theories of fluctuations and correlations in plasmas are reviewed with emphasis on the concept of the transition probability of a test particle. The simple cases of electrostatic correlations in uniform unbounded plasmas are considered first. It has been shown that the transition-probability approach reproduces an exact result obtained from a solution of the kinetic equations for stable quiescent plasmas. The transition-probability approach is then extended to the cases of inhomogenous unbounded correlations. Finally, we consider general cases of electromagnetic correlations in homogenous bounded plasmas. Various applicational examples are discussed; usefulness of the transition-probabilty approach is thereby discussed.