The aim of this paper is to analyse shared custody after the conflictual end of a marriage and the best interests of children and adolescents. It also aims to analyse the evolution of the family since its beginnings, to analyse the Constitution in relation to the creation of ECRIAD, Family Law and to focus on the issue of parental alienation. This is characterised when one of the parents demoralises the image of the other for the child or adolescent without justification, simply out of anger, for not accepting the end of the relationship, revenge, frustration and even for financial reasons. It also emphasises how shared custody works and how it deals with the origin of conflicts.