The paper reviews the results obtained from theoretical studies of the above-barrier particle interaction with crystal atoms. It is shown that in the motion of relativistic electrons and positrons in a crystal at a small angle to one of crystallographic axes quantum effects should manifest themselves in scattering; the radiation spectrum of above-barrier electrons has a maximum in the low frequency region and a high radiation intensity at the maximum; and the mean square of the multiple scattering angle in the crystal appreciably exceeds that in amorphous targets.