This book is about the technology of using fluid production or injection and pressure measurement signals from wellbores and relating those signals to the subsurface geology and subterranean reservoir properties. It is aimed at students of well-testing and practicing petroleum engineers or geoscience professionals for subsurface characterization and modeling. The topics include the art and science of well-test analysis, pattern recognition of rate and pressure signals, and a quantitative approach for estimating important subsurface geological parameters for subsurface aquifers and reservoirs containing oil, gas, and geothermal resources. The book is also particularly of value as a guide to asset managers actively developing unconventional reservoirs and CCUS.