The source rock is the beating heart of any petroleum system that is to exist in the sedimentary basins. It is defined as a fine-grained sedimentary rock that contains adequate quantities of organic matter of favorable quality (i.e., the type and thermal maturity). The critical aspect of any source bed is its cumulative expulsion potential, which is defined as the ultimate masses of oil and gas that can be expelled during the complete maturation of that source rock. The ultimate expellable potential is controlled by the source rock organofacies and the effective thickness. Viable hydrocarbon accumulations cannot form in the sedimentary basin unless the expellable masses exceed the cumulative masses lost during migration. This chapter provides a brief overview about the deposition and thermal evolution of the organic-rich beds in sedimentary basins. In addition, some guidelines are provided about the conventional techniques that are used for geochemical evaluation of organic-rich source rocks.