It is unanimously accepted that a theoretical approach of a system or phenomenon reveals new features and offers a deeper insight into the intimate mechanisms. In life science, this approach is mainly based on mathematical modeling, followed naturally by computer simulation. The chapter presented here tries to give the reader a comprehensive view over the main issues arising when attempting to build models of biological systems. A series of applications is shortly presented. The second half of the chapter is dedicated to one of the most interesting models: protein synthesis regulation. The example follows the classical steps: the scheme of the processes to be described, the set of differential equations and the results, including their possible interpretation.