The civil law tradition is the oldest and most prevalent legal tradition in the world today, embracing the legal systems of Continental Europe, Latin America and those of many African and Asian countries. Despite the considerable differences in the substantive laws of civil law countries, a fundamental unity exists between them. The most obvious element of unity is the fact that the civil law systems are all derived from the same sources and their legal institutions are classified in accordance with a commonly accepted scheme existing prior to their own development, which they adopted and adapted at some stage in their history. The civil law tradition was the product of the interaction among three principal forces: Roman law, as transmitted through the sixth century codification of Emperor Justinian; Germanic customary law; and the canon law of the Church, which in many respects derived from Roman law but nevertheless constituted a distinct system. Roman law is both in point of time and range of influence the first catalyst in the evolution of the civil law tradition. The history of Roman law is divided into two great phases. The first phase spans more than a thousand years, from the formation of the city-state of Rome to the codification of Justinian in the sixth century AD. During its long history, Roman law progressed through a remarkable process of evolution. It advanced through different stages of development and underwent important transformations in substance and form as it adapted to the changes in society, especially those derived from Rome’s expansion in the ancient world.