This paper reports that gas injection is one of the oldest methods used by engineers to improve oil recovery, and its use has increased recently. Much of the new expansion (especially in the U.S.) is coming from the nonhydrocarbon gases, nitrogen and CO{sub 2}. The full impact of CO{sub 2} flooding will be felt during the next several years since construction of CO{sub 2} pipelines into the west Texas area was completed in the 1980's and the larger floods are continuing to respond with good increases in tertiary oil production.