The present dissertation brings a reflection on the application of the principle of the best interest of the child in the interfamiliar conflicts of custody, searching its origin and raising some criteria that can guide its use, once there is not an express concept in the legislation. Our main goal was to verify if the referred principle was being considered or not in the decisions of the disputes of guard between the genitors, after the divorce or the separation. In order to accomplish it, we developed a qualitative and exploratory study at the Núcleo de Prática Júridica of the PUC/RJ, in which fourteen proceedings of custody in the period of 1997 until June of 2007 were analyzed. The analysis of these proceedings showed that the principle of the best interest of the child is not yet taken into consideration by some of the Law workers. The proceedings in which the social workers and psychologists participated we noticed that the adolescent/child were seen as the main characters (absolute priority) so they always got the chance to express their opinions (full protection) and therefore offered the means to point out the most suitable way to solve the cases (best interest). On the other hand, the judges and prosecutors of the analyzed proceedings did not seem to be concerned about the principle of the best interest of the child, their greatest concern was to put an end to the lawsuits making a deal with the parents, disregarding the details of the conflicts and the well being of the children and adolescents involved.