Book excerpt: Understanding the origin and fate of hydrocarbons in the subsurface was the major endeavor of organic geochemists during the second half of the twentieth century. They succeeded to the point where the deciphered interplaying of elements and processes paved the way for the revolutionary concept of the petroleum system, a unifying paradigm that plays an important role in decision making associated with oil and gas exploration. The chemistry and physics involved have been addressed in a quantitative way and integrated into the other aspects of petroleum geology, giving rise to the development of numerical basin modeling. This book has been designed to offer an overview of different aspects of the geochemistry of fossil fuels, in particular the functioning of a petroleum system. In this respect, it can be viewed as a foundation for approaching basin modeling. This book will be of interest to a large audience including specialists in the field, nonspecialist professionals, and undergraduate and graduate students.