A cohabitation is an institute of family law from which personal and property law effects arise, as well as the right and obligation of extramarital partners to mutual support, which consists of both personal and property effects of a cohabitation. The focus of this paper is on the legal regulation of property relations of extramarital partners. The first part of the paper gives a brief overview of the development of cohabitation in Europe and the development of cohabitation in Croatia. First of all, it is discussed how it came about to protect the regulation of personal and property effects of the cohabitation and in which countries the development of protection first appeared. Before property relations in a cohabitation, it was discussed what a cohabitation is in general, what are the preconditions for its establishment and how legal uncertainty arises in cohabitations. In the second part of the paper, the family law regulation of property relations in an cohabitation is discussed. We distinguish between two systems of regulating property relations in marriage according to Croatian family law, and these are legal and contractual. The paper will discuss both systems, and in particular the management of the matrimonial property and its dissolution. As there is no special legal term for property acquired during the cohabitation, and the term marital property is not suitable for designating property acquired in an cohabitation, the term "property of extramarital partners" will be used for this property. At the end of this paper, it is mentioned what the regulation of property relations is like after the termination of the extramarital union, the thoughts on the current legal system are concluded and the author's opinion on the protection of the extramarital union in Croatia is given.