Free intraperitoneal air after abdominal surgery is a confounding finding with uncertain significance. A diagnostic dilemma often arises as to its origin: does it merely represent residual postoperative pneumoperitoneum (PP), which will need no intervention, or does it indicate a complication such as an anastomotic leak or a perforation of the gastrointestinal tract. Residual PP is usually well tolerated, as it will be absorbed over time and requires no therapy. On the other hand, air escaping through a gastrointestinal tract perforation or leak usually represents an intra-abdominal catastrophe requiring urgent intervention. This intriguing subject has been dealt with quite extensively based on plain film radiography findings in the past 50 years, and has lately also been studied on CT. This review discusses factors influencing the prevalence of PP and its range of duration.