Supercritical carbon dioxide (SCCO2) has dissolving power comparable to that of liquids, is much more compressible than dilute gases, and has transport properties intermediate between gas- and liquid-like. This unusual combination of physical properties can be advantageously exploited in environmentally benign separation and reaction processes, as well as for new kinds of materials processing. This book articles summarizes the modifications and preparations of membranes that have been conducted in SCCO2.