This article describes how the State, which until then had consistently demonstrated the higher containment and deference towards the family circle and the omnipotence of the father, came to substitute itself to the paternal authority. With emerging new shapes of union and "kinship", the State's intervention became increasingly significant to the detriment of the father's authority, in accordance with its patria potestas (in civil law) and parens patriae (in common law) jurisdiction. Through the study of notions such as parental authority and child's interest, this article looks into how the State took advantage of sociological upheavals to further monitor family relationships. Such interference may take various forms and occurs both throughout the union and after the separation. Its aim is to protect, when situations of dysfunction and disputes occur, what constitutes the family foundation and perpetuates it: the child. The intermeddling of the State in family matters has been the subject of many critics, the most vocal of them being that it has substituted a State patriliny to the traditional family scheme, for the sake of the child's best interest. The author studies the role and the scope of the State patriliny within the family unit and after its dislocation. This article essentially refers to Québec civil law, while offering elements of comparison with family law developments in the common law provinces of Canada.