Air injection for recovery of conventional and heavy oils is a generally overlooked option that deserves a much greater level of consideration in the mix of IOR processes. Air injection for re-pressurization of reservoirs was first used in the early 1920s, but in spite of many excellent pilot and field scale projects, the process never assumed the application level that it deserves. The success of several air injection projects currently being operated by Continental Resources Inc. in the Williston Basin of North and South Dakota in the U.S. has confirmed that air injection is a viable IOR option which should be considered for both onshore and offshore applications. Air injection for IOR is termed as in situ combustion (ISC) when applied in viscous, heavy oil reservoirs, and as high pressure air injection (HPAI) when used in deep, light oil reservoirs. These designations represent the two ends of the spectrum of potential air injection projects. So, while there are many similarities in the operation of ISC and HPAI processes, there are some significant differences that must be understood when designing field applications.