In times in which, in view of tremendous currents of migration, there is controversy over relations towards foreign law - such as the law involving marriage to minors - it is worth taking a look back at history. Even in the Roman Empire, coexistence among peoples from different legal cultures was an everyday problem - whether due to the presence of foreign traders or because of Roman rule over foreign peoples. Did the Romans have any form of international private law comparable with ours - i.e., did they apply foreign law in some cases? If so, in which? This volume discusses these issues systematically, making use of literary sources as well, and alongside the fundamental question of international private law it also deals with individual principles such as 'locus regit actum' and 'lex rei sitae'. The book aims to offer an overall presentation of a problem that has long been familiar but has not yet been exhaustively discussed.