The International Union of Speleology (Union Internationale de Spéléologie, UIS) is a non-profit, non-governmental organization founded in 1965 in Slovenia (part of former Yugoslavia) on the initiative of the 4th International Congress of Speleology. Since 1953, these congresses are held every four years to promote interaction between academic and technical speleologists of different nationalities and with the purpose of developing and coordinating international speleology in all of its scientific, technical, cultural and economic aspects. The Union consists of member nations with voting rights, and each is represented by a delegate who represents all cavers and speleologists in its country. Until this book, the history of the International Union of Speleology was spread out in the minutes of the meetings and general assemblies, various UIS publications, and the proceedings of its International Congresses. Moreover, much of it was never written and was available only from the memories of past presidents, secretaries and other members of the UIS. In this book, the author presents the purpose of the Union and summarizes all its events through the first 50 years of its existence.