In this introductive chapter, basic concepts concerning natural gas seepage, including the terminology typically adopted for various types of seepage, the origins of gas (microbial, thermogenic, and abiotic), and the significance and implications of gas seepage for petroleum exploration, the environment, planetary geology, and astrobiology are presented. The types of surface gas occurrences that can be considered as “gas seepage” are also clarified, with special reference to the concepts of modern and fossil gas, and to the gas occurring in frozen, polar regions. The implications of natural gas seepage, both offshore and onshore, are summarised by tracing the history of its study via academia and the oil industry, and by arriving at the modern, comprehensive, and holistic view that guides this book. The arguments discussed in successive chapters are also briefly presented.