The institution of marriage is considered as one of the most important social institutions that traces its roots to the period of Roman law, especially in the sense of marriage as contracted in the societies based on Judeo-Christian tradition. Although this institution is still very important in the modern age, its importance is not as high as before due to the existence of an alternative union called cohabitation, with equal legal effects, in which people can realize their need for closeness. Just as it can be said that marriage has its legal foundations in Roman law, it can similarly be said that the contemporary cohabitation shares similarities with the corresponding union in the Roman legal tradition, called concubinage. It is difficult to determine the precise time of the establishment of concubinage. The institute was hardly mentioned until Plaut's literary texts, while it acquired legal relevance only with the marital legislation of Emperor Augustus, and then only incidentally and indirectly. Because of that, the jurists of the classical had a task of interpreting the limits of concubinage and to give their understanding of the concept of concubinage and who would be allowed to be in such a union, considering the Augustus’ laws which introduced many prohibitions related to marriage. The prohibitions paved the way for concubinage as a community that was an alternative to marriage, but did not share legal effects of marriage. Namely, the concubine and the children born in concubinage did not have the same rights as the wife and the children born in marriage, which mostly related to inheritance. It was only in the post-classical period that the smperors introduced institute which would allow children born in concubinage to have the right to inheritance, the institute of legitimation. Although at first it may seem that concubinage in the Roman legal tradition and cohabitation in modern Croatian law are similar, they are quite different in terms of legal effect. Although both institutes are monogamous permanent unions of a man and a woman, the similarities end there. There are differences in the reasons for entering such unions; while in Roman law such union was widely used due to many prohibitions related to marriage imposed by the laws of Augustus, in the Republic of Croatia people enter into a cohabitation because they want to. Differences are also visible in the legal effects produced by these institutes. In the Republic of Croatia, all illegitimate children are equal to children born in marriage (expect when establishing paternity), which means that they have the same right to maintenance and inheritance rights as children born in marriage. Likewise, an extramarital partner is equal to a spouse in terms of property relations, maintenance and inheritance rights.