With the development and changes in the social core, the law can not remain stagnant, it is always up to him to innovate and develop, otherwise it will lose the ability to resolve conflicts of interest and respond to the desires of society. Today, marriage is not the only way to constitute a family, existing the most diverse forms of family, as the stable union (central theme of this work), single-parent families, homoafetivas, etc.. Thus, it is up to the law to always seek the protection of such institutes in its legal body. The theme is of great relevance, since the number of cases of stable unions grows every day and it is necessary to clarify the doubts that surround it, always in the light of constitutional principles. In view of the existence of the possibility of partners instituting a cohabitation contract to deal with the property rules that will govern the union, the following question arises: what will be the property regime to be observed in the absence of such a contract?