The Spanish 1978 Constitution establishes a complex state. In constitutional terms, it is not defined as a federal state, but rather as a unitary state with significant scope for political decentralization. This complex constitutional arrangement is in response to both internal and cultural features, and to a search for greater administrative efficiency. The pluri-national character of the state is the subject of fierce debate in Spain, while the way in which certain minority nations (Catalonia and the Basque Country) are accommodated is a permanent source of friction. This paper seeks to provide an overview of this complex political/constitutional situation by analysing the historical, political and legal developments that have occurred during the last 40 years, with a focus on the last developments of the Catalan crisis. Although Spain is markedly asymmetric in political and identity terms, this asymmetry is not adequately reflected in Constitutional Law. In addition, the ongoing tension between unionism and separatism in some regions poses significant challenges to the Spanish constitutional system as a whole, in particular, through the pro-independence process in the autonomous region of Catalonia.