Vacuum based techniques are characterized by the presence of residual gas. Depending on the affinity of the residual gas to the growing film material, chemical reactions may be possible. Residual gas based impurity incorporation during thin film growth has been reported previously.@footnote 1,@@footnote 2@ Here, the state of the art in residual gas - growing film interactions is reviewed. Sources for residual gas incorporation as well as incorporation mechanisms are described. Furthermore the effect of impurity incorporation on the film structure and film properties are discussed. @FootnoteText@ @footnote 1@ J.M.Schneider et al, Appl.Phys.Lett. 74, 200 (1999). @footnote 2@ J.M.Schneider et al, Appl.Phys.Lett. 75, 3476 (1999).