Monks in Byzantine times (330-1453 AD) often expressed their faith with extreme manifestations of behaviour, such as living on a high column (stylites), on a tree (dendrites) or in crowded urban centres of the empire pretending to be fools for Christ's sake. These Holy Fools exposed themselves to the ridicule and the mistreatment of the citizens, being protected, however, by their state of insanity to mock and violate moral codes and social conventions. The official Church barely tolerated these religious attitudes as promoting deviations from standard orthodoxy, and the Quinisext Ecumenical Council (592 AD) judged them as dangerous and formally denounced the phenomenon. The two most famous of them in Byzantium were Symeon of Emesa and Andrew of Constantinople, whose lives constitute unique testimonies to insanity and the simulation thereof. The survival and transplantation of the Holy Fools in Russia, called "yurodivye", where they met widespread acceptance, confirm their appeal in specific geographic areas and their endurance over time. We attempt to approach the symbolism of holy lunacy and to analyse the personality trends of these "eccentric" saints.