Oil and gas production rates from a well often undergo a declining behavior over time. Well productivity is a complex process that is a function of the hydrocarbon reservoir subsurface properties related to the fluids in places and the hosting environment. It is also related to the wellbore flow conditions from the reservoir to the surface. Well testing is an important technology that is frequently used in the industry. This technology consists of flow diagnostics (rates and pressure) to evaluate a well productivity or injectivity performance such as skin factor, non-Darcy effect, and storativity. It is also used to acquire insights about the reservoir properties such as connectivity, heterogeneity including fractures, flow regime, and drainage area. This course covers the fundamentals of well testing and discusses real field applications. The course includes : 1) fundamentals of flow in porous media; 2) introduction to decline-curve analysis; 3) Buildup-test analysis of slightly compressible fluids; 4) Analysis of oil and gas well flow and buildups tests; 5)Well-test in hydraulically fractured wells; 6) Well-test in naturally fractured reservoirs; 7) Interference and pulse testing; 8) well testing in unconventional reservoirs. Note: students are expected to have at least basic familiarity with: Multi-phase flow in porous media, reservoir engineering, and programming in Matlab or Python.