  The effect pointed out by A. B. Migdal in the 40's (hereafter named Migdal
effect) has so far been usually neglected in the direct searches for WIMP Dark
Matter candidates. This effect consists in the ionization and the excitation of
bound atomic electrons induced by the recoiling atomic nucleus. In the present
paper the related theoretical arguments are developed and some consequences of
the proper accounting for this effect are discussed by some examples of
practical interest.
