The article gives a survey of Th. Mommsen's main works and demonstrates the inseparablilty of his historical and legal studies based upon a profound knowledge of various types and kinds of sources and their critical (but never hypercritical) analysis. The author presents some new data on Mommsen's activities. They include Alexander Demandt's notes of Mommsen's readings, reflecting the contents of the missing 4th volume of his Romische Geschichte, as well as the recently published materials about Mommsen's visit to Sardinia, among them his correspondence with his colleagues and pupils connected with the preparation of the CIL. The author shows that in spite of the new sources, which have radically changed our idea of the earliest period of Roman history, Romische Geschichte, due to its logical structure, appealing and imaginative style, his articles on Roman history, his publications and studies of Roman inscriptions, as well as his research in the field of Roman law (especially his Romisches Staatsrecht), remain indispensable for the study of the history of Rome.