This review surveys the quantum theory of electromagnetic radiation for atomic systems.
In particular, a review of current theoretical studies of multiphoton processes in one and
two-electron atoms and highly charged ions is provided. Grounded on the quantum elec-
trodynamics description the multiphoton transitions in presence of cascades, spin–statistic
behaviour of equivalent photons and influence of external electric fields on multiphoton in
atoms and anti-atoms are discussed. Finally, the nonresonant corrections which define the
validity of the concept of the excited state energy levels are introduced.