PTA is a Reservoir Engineering tool when we identify reservoir flow regimes. Flow regime development, or lack thereof (obscured, absent, transient), is crucial to analytical confidence levels. Vertical Radial flow was the norm fifty years ago: easy discovery, conventional, homogeneous. Horizontal wells now complicate matters in unconventional formations (shale) with multiple regimes: from Storage to Vertical-Radial to Linear/Elliptical to Horizontal-Radial to Pseudo-Radial flow – but we rarely see them all clearly. In other plays, Linear-Channel flow is particularly disappointing for long-term gas deliverability predictions, and Bilinear reservoir flow is particularly difficult for material balance and reserve estimates. Composite flow regimes can help interpret heterogeneous formations (net pay variations), while dramatic changes in flow regimes can help identify liquid interfaces (viscosity changes).