One of the forces responsible for the primary recovery of hydrocarbon, is the encroachment of a large pool of water body underlying the hydrocarbon accumulation in the reservoir structure. In the evaluation of hydrocarbon reservoir performance, it is paramount to accurately determine the amount of water encroaching into the reservoir whose value is dependent on the water viscosity, the permeability of the rock in the aquifer and the cross-sectional area between the water zone and the region where the hydrocarbon is accumulated. There are several analytical aquifer models presented in the past to estimate the amount of water encroaching into reservoirs, some of which can be applied to linear or radial aquifer, bottom aquifer and/or edge water, finite and/or infinite-acting. Van Everdingen & Hurst method requires the principle of superposition which is a tedious exercise, but it provides an exact solution to the radial diffusivity equation and can be applied at the early stage. The Carter-Tracy aquifer models can be applied to both finite and infinite-acting aquifers, it can be applied to both radial and linear aquifers and also applies to edge-water drive reservoirs only. Fetkovich model applies to both radial and linear aquifers, finite-acting aquifers, edge-water and bottom-water drive reservoirs. Thus, to understand the various aquifer models, several solved example questions and exercises are presented.