The physical mechanism of the Landau damping is discussed and the general expressions for the transformation of energy and momentum due to Landau damping in time are derived. The presented theory is not limited to a small decrement to the wave frequency ratio and it removes some artificial assumptions and discrepancies involved in older theoretical models. The perturbation of particle velocities, caused by self-consistent electric fields, is shown to be formed by a superposition of free and forced coherent oscillations propagating in space. Thus, the transformation of energy and momentum may be treated as a process of phase interference between these two types of oscillations, in which the evolution of the forced oscillations is governed by the energy and momentum balance equations. (J.U.) 13 refs.