A review is given of the properties of highly excited atoms placed in an electromagnetic field. The probabilities of bound-bound and bound-free transitions between quasiclassical atomic states, and also approximate selection rules for such transitions, are examined. The properties of the dynamic polarizability of highly excited states of atoms are investigated. Quantum-mechanical ionization mechanisms (multiphoton and tunneling) are discussed for highly excited states. Much space is devoted to the stochastic dynamics of classical atomic electrons in a varying monochromatic electromagnetic field. The threshold electric field for the stochastic motion of an electron and the ionization of an atom is given as a function of the field frequency, its polarization, and the principal quantum number of the atomic state under consideration. The influence of orbital angular momentum of the state from which ionization takes place on the stochastization process is discussed. Classical diffusion ionization of a highly excited atom in an electromagnetic field is considered within the framework of classical mechanics and the quasiclassical quantum-mechanical approximation. The validity of classical mechanics in relation to the properties of highly excited atoms in an electromagnetic field is examined. The realization of quantum and quasiclassical ionization of highly excited atoms is considered. The final part of the review analyzes experimental data on the behavior of highly excited atoms in a radiofrequency field. Comparison of experimental data with the theory given in this review has demonstrated good agreement between them.