Gas separation by membranes is a rapidly growing field. It has been recognized as one of the most recent and advanced unit operations. Most commercial gas separation membranes in use today are based on the defectfree and high performance asymmetric membranes. Since the membrane is the heart of the gas separation processes, research and development in this field has been continued extensively. This paper reviews the route that has been taken in developing defect-free and high performance asymmetric membranes for gas separation processes through the combine effects of primary phase inversion process and rheological factors. The former controls general morphology of membrane whereas the latter further affects molecular orientation in membrane. The development of these aspects represents a major breakthrough in membrane technology.