This book is designed to be a technical reference for engineers and technicians who interpret well tests. For each case examined, it describes the most suitable interpretation methods, such as traditional methods, type curves, computer-generated type curves, and interpretation using derivatives. It cites the equations used to interpret results, while avoiding detailed mathematical discussions. Nevertheless, as numerous references are provided, those who wish to can trace the solutions back to their origins. The book also indicates the conditions under which different methods are useful, and is illustrated with a large number of figures and practical examples. The text shows how the interpretation of a well test varies according to the type of reservoir limits (e.g., faults, channels, secant faults, constant pressure limits, closed reservoir, etc.), the well reservoir configuration (e.g., partial penetration wells, inclined wells, artificially fractured wells, horizontal wells, injection wells, etc.), and the nature of more complex reservoirs (fissured reservoirs, two-layered reservoirs). It indicates which methods to use when the fluid in a well is a gas or a polyphasic fluid. Lastly, it deals with tests applied to multiple wells, including interference tests and pulse tests.