On 9 June 2017, the Government of Catalonia announced the celebration of a referendum on self-determination to be held the next 1 October 2017. The Catalan call triggered a new political and legal quarrel with the Spanish government that was inexorably opposed to recognising the right of self-determination of Catalonia. The political clash between the State and the autonomous community opened several questions on the use of referendum on self-determination without the authorisation of the State. This chapter scrutinises the Catalan referendum on independence analysing in detail both the Spanish arguments to abort the voting (considered as an unconstitutional act that breached the rule of law and a criminal offense) and the Catalan thesis to hold the plebiscite (Democratic principle and the exercise of the human right of self-determination. The chapter follows by critically analysing the legitimacy of secession referenda in constitutional democracies and its suitability, or not, to settle territorial disputes where national minorities are involved. The conclusion summarises the chapter’s main arguments and raises questions for future research on secession referenda.