This article explains how the creation of the federal idea is dependent upon a specific time and space. The federal idea in the Constitution of the United States is different from the idea of Germany in 1871. It responds to different problems and needs. Nevertheless, the conceptual construction of what a federal state means and the corresponding juridical design were taken out of these particular cases, therefore were very rigid and did not take into account differences in the particular conditions of society. This conceptual construction is re-evaluated in the second half of the XXth century as a more general category of federal arrangement. This category is more flexible and comprehensible of the reality of the context where the federal idea develops, as well as it is able to take into account the specific
needs of the population. With this framework, I analyze in this article the Spanish case; I seek to establish the contemporary elements of a federal state, and how the idea evolves in a real environment.