Welltesting & Pressure Transient Analysis (PTA) are Vital Tools for Upstream Oil & Gas Strategy. Welltesting and PTA are disciplines of Reservoir and Production Engineering. Reservoir engineering because: we identify insitu flow regimes, interpret matrix and fracture flow characteristics, identify boundaries and faults, infer drive mechanisms and liquid contacts, and quantify permeability or flow capacity. Production engineering because: we test for near wellbore condition (skin); calculate inflow capacity; and evaluate tubing and pump performance. PTA is also an economics tool when we produce an inflow deliverability forecast, a reserves tool when we develop material balance analyses, a completions tool when we qualify induced fracture stimulation treatment (frac’) effectiveness, a geological tool when we deduce boundaries or faults, and an exploration tool when we analyze a DST. Bottom line, PTA is a management and business decision tree tool, a valuable piece of the complex hydrocarbon recovery puzzle, where everything is measured indirectly and thus requires specialist interpretation. PTA corroborates with seismic, core, logs, maps, cross-sections, and PVT to provide us our best guess scenario. Welltesting is responsible due diligence for upstream E&P business practices and management decisions.