Against a background of increasing energy demands and diminishing reserves of fossil fuels there is a clear need to intensify the search for conventional fuels and also to identify and develop new and unconventional energy technologies.  But when trying to obtain a relatively complete picture of the level and distribution of energy resources around the world, information on petroleum reserves (oil and gas) is particularly difficult to obtain.  The Fifth IIASA Conference on Energy Resources focused on conventional and unconventional sources of natural gas in an attempt to fill a major gap in the available data base, in recognition of the great and increasing importance of gas in world energy supply. The 31 papers in this volume are grouped in seven sections.  The book opens with papers on the importance of natural gas in world energy supplies, followed by a section on the origin of natural gas and methods and models for assessing the location and extent of gas resources.  The third section reviews past estimates of world gas resources while the fourth presents national and regional studies of two major producers, the United States and the Soviet Union.  Technological, economic, an institutional aspects of gas utilization are examined in the fifth section, which is followed by a status report and future prognosis for gas located in geopressured zones.  The book closes with papers on "unconventional gas", syngas, and biogas.