A worldwide interest exists today in drilling horizontal wells to increase productivity. The primary focus of this book is to derive productivity formulae for horizontal wells in anisotropic reservoirs using straight forward mathematical techniques. For horizontal wells, steady state and unsteady state pressure transient testings are useful tools for evaluating in-situ reservoir and wellbore parameters that describe the production characteristics of a well. Formulae presented in this book are based on analytical solutions to the diffusivity equations in petroleum reservoir engineering. This book is arranged so that it can be used as an engineering mathematics textbook for graduate students in petroleum reservoir engineering, and can help the students have a complete understanding how mathematics can solve complex problems in reservoir engineering. This book is also a reference book for research scientists in petroleum industry and practicing reservoir engineers.