The pressure build-up equation derived by Horner is applicable to wells that produce at a constant rate. However, as a constant flow rate does not exist in practice, the Horner equation is not expected to yield accurate results. Because of this limitation, an alternative method must be used to account for a variable production-rate history. The principle of superposition enables the generation of pressure behavior solutions for variable-rate cases based on the constant-rate pressure behavior solutions. The superposition solution and its application in pressure build-up analysis are herein presented and are compared with the Horner method for different interpretation cases. Finally, the accuracy and reliability of the calculated results are discussed.