Polymers are widely used as membrane material to perform the separation of various gaseous mixtures due to their attractive permselective properties and high processability. Although the development of new robust materials is fundamental to enlarge the economic competitiveness in aggressive environments and severe operating conditions, the correct combination of multiple membrane stages in appropriately designed systems can improve significantly the performance, achieving the separation target by using existing materials. In fact, a single membrane stage requires the lowest membrane area but also produces the lowest purity in the permeate stream, whereas multiple membrane stages provide higher purity values with higher membrane surface areas. According to the specific transport properties of rubbery and glassy polymer membranes, it is possible to select an appropriate material in a sequence of membrane units that maximises the purity and the recovery of a species by means of a right share of the separation load on each stage. In this framework, some examples of gas separations of industrial interest are discussed to support this methodology.