Hydrogen (H) permeation barrierPermeation barrier is widely employed to avoid H entryH entry in metals. In particular, oxideOxide coating coatingCoating has been extensively studied to retard the H entryH entry and permeationPermeation. However, the effects of the coatingCoating are easily lost by its tiny cracks and holes, and hence do not depend on the thickness of the coating. Since thicker coatingCoating tends to re-crystalize and introduce stronger stressStress between the coating and the substrate, the thin and tightly bonded coatingCoating would be better. Often used are thin amorphous like oxides of Al, Be, Cr, and Y. Some carbidesCarbide and nitrides are also used as the coating. Recently it has been indicated that the double membraneDouble membrane without evacuation of the space between the membranes could strongly reduce the permeationPermeation ratePermeation rate owing the oxygenOxygen remained in the space. At present, it is quite hard to obtain the permeationPermeation reductionReduction factor (PRF)Permeation Reduction Factor (PRF) over 104, and further R&D is needed to get the permeationPermeation barrierPermeation barrier having integrity and low PRF.