As in other countries, the Spanish upper chamber is facing harsh criticisms. It has failed to fulfil its constitutional task as a chamber of territorial representation. Notwithstanding a number of proposed reforms, the Senado has remained almost unchanged since its creation in 1978. So why is it so difficult to restructure this chamber? This research aims to explain the impasse of the reform of the Senate through evaluating three approaches. After stressing the qualities and defects of the legal inheritance and party bargaining frameworks, this article argues that the joint-decision trap perspective can help to understand the two-fold dynamic of institutional obstruction and incremental change that has affected the Spanish Senate for the last 20 years.