The basic physical principles underlying the technique of recoil implantation are discussed in some detail. The number of recoils as a function of recoil energy passing from an evaporated layer into the substrate is calculated for a particular case; and the final penetration distribution within the substrate is compared with that expected for a direct implantation. The number and distribution of unwanted contaminant impurities which inevitably recoil into the substrate during direct implantation through an oxide layer, is also discussed.