H2O vapor is an undesirable component of natural gas. It takes up space in the pipeline and provides no fuel value. In higher concentrations it can condense into liquid water in the pipeline and cause corrosion, especially in the presence of carbon dioxide or H2S. Liquid water can also cause damage to the equipment utilizing the gas, for example to turbines. Because of this, most gas transfer tariffs include a limit on the acceptable concentration of H2O in the gas stream. This paper reviews the devices that can be used in the field to determine the amount of water vapor present in a natural gas stream