Membrane gas separation has potential for wide scale implementation in CO2 capture, as part of the Carbon Capture and Storage (CCS) strategy proposed for climate change mitigation. CO2 selective membranes are commercially available and membrane processes for natural gas sweetening have been proven. The majority of CCS will be implemented on post-combustion scenarios, which is carbon capture from low pressure flue gas. This presents unique challenges to gas separation membranes, requiring process design strategies that generate a pressure ratio across the membrane for efficient separation, while minimising the energy duty of the process to ensure the economics are competitive. This chapter reports on design strategies for gas separation membranes that enable membranes to undertake post-combustion capture from flue gas, and investigates their economic competitiveness. Processes based on membranes only, such as membrane cascades with recycle, have been demonstrated to be uncompetitive for post-combustion capture compared to alternative technologies. However, the design features developed in these processes to generate the membrane pressure ratio are important for low pressure applications. Hybrid processes combining membrane gas separation with another separation technology have been simulated to be economically competitive, depending on the application. Importantly, membrane gas separation with cryogenic separation is a viable process design. This is because of the membrane stage’s ability to exclude the majority of nitrogen from the cryogenic separation, reducing its energy duty. The ability to recycle CO2 through a combustion burner, and hence increase the flue gas CO2 partial pressure, is a design feature that significantly reduces the cost of capture for flue gas emitted from coal-fired power stations. For many CCS applications, developments in membrane process designs are as important as developments in the membrane’s CO2 permeability and selectivity, since the economics of many of these processes are dependent on the gas processing equipment rather than the membrane. Hence, membrane process designs and strategies developed to minimise the energy duty are vital for gas separation membranes in the CCS field.