Two main elements characterize a country as federal and the development of its federal system. The first one is directly connected to competences and how regions may or may not have powers over public policies. The second one focuses on the fiscal arena, and how regions generate their own income and share it with the central government. This paper describes from a historical point of view the different phases that the Spanish federal process has followed since its beginning in 1978 up to the last reform in 2016. These phases are related to the composition of both the regional and the national governments. It is therefore is important to investigate the connection between changes in these compositions and how they may have affected the phases of decentralization. Employing the new institutionalism paradigm and seeking for critical junctures in the different moments, we show how national governments are more important in shaping this process than regional ones.