For which reason must we obey authority or we should refuse obedience, this is a universal concern which arises in different periods of time, religions and cultures. This collective work analyses this and other issues, focusing on the medieval period and in the Greek Byzantine, Christian Latin, Jewish and Islamic traditions. What makes political power legitimate? Without legitimation, subjects will not accept power, and, since religion permeated medieval society, religion became foundational to philosophical legitimationsof political power. In 2013, the XIX Annual Colloquium of the International Society for the Study of Medieval Philosophy took place in Alcalá de Henares, one of the Medieval centers of political debate within and between Jewish, Christian and Muslim communities. The members of these communities all shared the common belief that God constitutes the remote or proximate cause of legitimation. Yet, beyond this common belief, they differed significantly in their points of departure and how their arguments evolved. For instance, the debate among Western Christians in the conflict between secular power and Papal authority sowed the seeds for a secular basis of legitimacy.