  A survey of physical parameters and of a ladder of various regimes of
laser-matter interactions at extreme intensities is given. Special emphases is
made on three selected topics: (i) qualitative derivation of the scalings for
probability rates of the basic processes; (ii) self-sustained cascades (which
may dominate at the intensity levels attainable with next generation laser
facilities); and (iii) possibility of breaking down the Intense Field QED
approach for ultrarelativistic electrons and high-energy photons at certain
intensity level.
