The primary aim of the book is to present the basic physics of reservoir engineering, using the simplest and most straightforward of mathematical techniques. It is only through having a complete understanding of the physics that the engineer can hope to appreciate and solve complex reservoir engineering problems in a practical manner. This is developed with a homogenous unit system with useful formulas expressed in practical units. Material balance is discussed extensively for oil and gas reservoirs. Attention is given to the importance of the aquifer match before starting a reservoir simulation. Associated gas reservoir development issues are presented. The last chapter is devoted to reservoir simulation. The Theory focuses on the modern tools used in the industry. Calculation techniques are explained to help the user to master the algorithms and optimize the management of the reservoir study. Numerous references are provided to guide the students for further reading. The book content will help students in their first approach to reservoir engineering and professionals to familiarize themselves with modern techniques.