This book contains the proceedings of NATO Advanced Study Institute, 'Underground Storage of Natural Gas - Theory and Practice', which was held at The Middle East Technical University, Ankara, Turkey during 2-10 May 1988. Underground storage is the process which effectively balances a variable demand market with a desirably constant supply provided by pipelines. Storage reservoirs are the unique warehouses designed and developed to provide a ready supply of natural gas in response to high, peak demands during cold weather. The natural' gas is injected into the underground storage environment when the market demand falls below the supply available from the pipeline. It is withdrawn from the storage reservoir to supplement the steady supply provided by the pipelines whenever the demand exceeds the supply. The overall wellbeing of the entire western world in general and of the NATO member count ries in particular depend critically upon having sufficient energy resources. Of over 80 quad Btus of energy consumed each year in the western world, about 30~ comes from natural gas, a figure only exceeded by oil. The technology related to supply and demand of natural gas has been in the focus of long range energy planning during the last decade in Western Europe. In view of recent developments related to natural gas in Europe and Turkey, an "Advanced Study Institute" programme in Turkey on underground storage of natural gas was deemed particularly relevant and timely.