Gas purging, a process of displacing one gas by another gas, occurs on a routine basis in the natural gas industry when pipelines are purged into and out of service. In a project sponsored by the Gas Research Institute and in cooperation with the American Gas Association (A.G.A.), the purging practices as outlined in the A.G.A.’s Purging Principles and Practices manual were reviewed because many of today’s pipeline purging operations occur under conditions not addressed directly in the manual. The program focus is on the purging procedures outlined in Chapter 8 of the manual entitled “Gas Transmission and Distribution Pipes.” The technical objective of the Project was to develop an understanding of the scientific principles upon which safe, practical purging practices can be based. Direct displacement and inert gas slug purging operations are explained in terms of dispersion and mixing parameters and their relationship to the gas velocity. Field data compared to the results of an analytical mixing model. Computer software for planning safe and cost-effective pipeline purges has been developed. Finally, recommendations for revising Chapter 8 of the A.G.A. manual are presented.