  The present paper is a review of the phenomena related to non-equilibrium
electron relaxation in bulk and nano-scale metallic samples. The workable
Two-Temperature Model (TTM) based on Boltzmann-Bloch-Peierls (BBP) kinetic
equation has been applied to study the ultra-fast(femto-second) electronic
relaxation in various metallic systems. The advent of new ultra-fast
(femto-second) laser technology and pump-probe spectroscopy has produced wealth
of new results for micro and nano-scale electronic technology. The aim of this
paper is to clarify the TTM, conditions of its validity and non-validity, its
modifications for nano-systems, to sum-up the progress, and to point out open
problems in this field. We also give a phenomenological integro-differential
equation for the kinetics of non-degenerate electrons that goes beyond the TTM.
