The relations between petroleum and source rocks can be understood only if we know the important geochemical and geological data of both systems. Therefore, in the first two sections, the latest results of oil and sediment research are discussed. The third section deals with migration, which may be interpreted as the connecting link between crude oils and their source rocks. In the light of the data presented, it is then shown that oil genesis and migration are very closely related to the development of a basin and that one source rock can deliver a whole series of different crude oils. At the beginning of this series are the heavy petroleums and at the end the light ones. This development is basically the result of the progressive thermal degradation of the organic material, which was finely disseminated throughout the source rocks.