The application of membranes for gas separation is a fairly young technology compared to the use of membranes for liquid separation. Although the basic theoretical principles were partly understood and date back to the early nineteenth and twentieth century with Fick’s law (1855), osmotic pressure (Van t’Hoff, 1887 and Einstein 1905), and membrane equilibrium (Donnan 1911), it was not until around 1950 that theories for gas transport through a membrane were presented and later further developed (the pore model by Schmid in 1950 and Meares in 1956 and the solution-diffusion model by Lonsdale in 1965) . The breakthrough for industrial membrane applications came with the development of the asymmetric membranes achieved by Loeb and Sourirajan around 1960 . These membranes were developed for reverse osmosis and consisted of a very thin dense top layer (thickness < 0.5 µm) supported by a thicker porous sublayer; hence, the ux, which is inversely proportional to the selective membrane thickness, could be dramatically increased. The work of Loeb and Sourirajan resulted in the commercialization of the reverse osmosis process for desalting of water and had also a major impact on the further development of ultraltration and microltration processes. The development of gas separation membranes is based on their achievements, and about 20 years later (~1980), the work of Henis and Tripodi made industrial gas separation economically feasible. They developed further the technique of depositing a very thin homogenous layer of a highly gas-permeable polymer on top of a porous asymmetric membrane, ensuring that pores were lled so that a leak-free composite membrane for gas separation was obtained. The rst major development was the Monsanto Prism® membrane for hydrogen recovery from a gas stream at a petrochemical plant . Within a few years, the Dow Chemical Company was producing systems to separate nitrogen from air and Cynara NATCO Group and Separex™ UOP LLC systems for the separation of carbon dioxide from natural gas. These rst membranes were all composite membranes where a very thin nonporous layer with high gas permeation rate (usually polysulfone or cellulose acetate) was placed on a support structure for mechanical strength-later, also other techniques for membrane formation were developed (i.e., interfacial polymerization, multilayer casting and coating).