The potential uses for hydrogen permeable gas separation membranes are introduced, and a simple mixed protonic-electronic conductor theory for calculation of flux rates is given. Example permeations are derived as a function of pressure gradients for different defect models, and the results of the very few available experimental studies of hydrogen permeation through oxides are discussed briefly. Strategies for new candidate materials are outlined. Indications of hydride ion transport in oxides under reducing conditions are introduced, and the behaviour and possible role of hydride defects in oxides is discussed.