Vacuum based synthesis techniques are characterized by the presence of residual gas. It is well known that the residual gas in high vacuum mainly consists of water. Depending on the affinity of the residual gas to the growing film material, incorporation during thin film growth has recently been reported.@footnote 1@ Here, a review on residual gas - growing film interactions is presented. Sources@footnote 2@ for residual gas incorporation as well as incorporation mechanisms@footnote 1@ are described. Furthermore the effect of impurity incorporation on the film structure and film properties is discussed.@footnote 3@ @FootnoteText@ @footnote 1@ J. M. Schneider, A. Anders, I. G. Brown, B. Hjorvarsson, and L. Hultman, Applied Physics Letters 75, 612 (1999). @footnote 2@J. M. Schneider, A. Anders, G. Yushkov, Applied Physics Letters 78, 150 (2001). @footnote 3@J.M. Schneider, K. Larsson, J. Lu, E. Olsson and B. Hjorvarsson, Applied Physics Letters 80, (2002) 1144.