The subject of this paper is an analysis of the institute of referendum in the Spanish constitutional system. The Constitution of Spain regulates several forms of the referendum, including the consultative referendum, referendums on amending the Constitution and various types of regional referendums relating Spanish Autonomous Communities. The paper draws attention, inter alia, to several controversial aspects relating mainly the consultative referendum, which are also in the spotlight of Spanish Constitutional Lawʼs doctrine. These controversies are centred mainly on the issue of legally binding force of the consultative referendum results. The analysis is completed by the comparison of selected issues of the institute of referendum in Spain on the one hand and in Slovakia on the other hand. As it will be indicated, some partial issues of Spanish constitutional regulation of the referendum should be inspiring for the Slovak Republic.