Considering the number of UNESCO Global Geoparks, Spain is the second country in the world and the first country in Europe. This paper describes the historical development and the current reality of Spanish Geoparks with a geographical, geological, administrative and sociological analysis and explains the main reasons that have contributed to the successful spread of Spanish UNESCO Global Geoparks. This analysis describes also the main challenges for the future and introduces a fundamental question that will have to be discussed in many countries with an important number of Geoparks: the criteria to define a sustainable number of UNESCO Global Geoparks (UGGp) at the national level. The representativeness of the national geological main features and its geological heritage in UGGp is one the main factors that can contribute to defining a well-balanced and impartial strategy for a sustainable growth. Thus, the relevance of a national geosite inventory rises up as one of the important tools to organize a coherent national distribution and to help on the definition of the boundaries of UGGp.