The lack of reservoir pressure data has always posed problems in production planning and oil recovery optimization. Subsurface reservoir pressure data acquisition through intrusive well intervention method might lead to loss of production and increased risk to the well. An alternative to well intervention is to employ transient flow modeling technique to estimate the reservoir pressure, as presented in this book. A mathematical workflow for the transient flow model was devised to simulate the hydrocarbon fluid redistribution in a well, from steady state flow until the fluid columns were fully segregated upon equilibrium over the well shut-in period. The salient features of this transient flow modeling technique were described, including simulation of reservoir fluid influx, gas-liquid interface movement, wellhead and bottom-hole pressure build-up over the fluid redistribution period during well shut-in. This transient flow modeling technique provides an alternative mean for petroleum engineers to obtain bottom-hole data without well intervention. Hence provides insight into well performance, improves reservoir management and decision making.