Citizenship is a specific legal relationship, permanent according to its character, based between a natural person and a sovereign state, on the basis of which certain rights and duties arise. We primarily associate citizenship with the principles of acquiring citizenship, namely: the principle of blood relationship (ius sanguinis), the principle of territory (ius soli) and the principle of residence (ius domicili). The aforementioned principles are the basis for acquiring the citizenship of most countries, including the Republic of Croatia. In the Republic of Croatia, the Law on Croatian Citizenship is the fundamental act that regulates the issue of citizenship, which gives detailed and precise answers to all questions related to citizenship. However, the Constitution of the Republic of Croatia, as the fundamental and highest regulation in the Republic of Croatia, stipulates: "Croatian citizenship, its acquisition and termination shall be regulated by law." A citizen of the Republic of Croatia cannot be expelled from the Republic of Croatia, nor can his citizenship be revoked, nor can he be extradited to another country, except when a decision on extradition or surrender made in accordance with an international agreement or the acquis of the European Union must be carried out. Although the paper also deals with European citizenship and the citizenship of certain Balkan countries, the focus is on the citizenship of the Republic of Croatia, to which we attach the travel documents with which a Croatian citizen proves his identity and Croatian citizenship.