Interest in the use of membranes to produce oxygen from air dating back to 1881 is reviewed. The development of the commercial interest by major worldwide industrial gas suppliers, and the current state of commercialization of membranes for production of gaseous nitrogen are summarized. Significant technology developments which occurred over the last decade are described and their impact on the development of commercial air separation membrane systems are illustrated. Specifically, the impact of ultrathin barrier layers, elimination of barrier layer defects, high flow countercurrent control of membrane contaminants and cost effective process cycles are discussed. Commercial experience with air separation membranes is summarized. Commercial systems are described, and experience with membrane life and system/operation are summarized. Performance growth in both number and size of membrane systems installed is presented. Comments on future directions for both membrane technology and applications in air separation are included.