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The conception of a "perturbed atom" that is an atom under simultaneous action of a number of perturbations in astrophysical and laboratory plasmas is developed. These perturbations are electric and magnetic fields, the Planck radiation field, plasma oscillations, collisions of an atom with other particles, effects of stochastic peturbations on an atom with a dense medium. The aim of this review is to solve fundamental problems connected with these phenomena. The main approach to these problems involves quasiclassical or pure classical methods which have revealed great potential in recent years. Much attention is devoted to a description of highly excited (Rydberg) atomic states. The presentation of this review is sufficiently detailed to provide a useful reference text for researchers.