Citizenship represents a special legal relationship between the citizen and the state. Throughout history, Croatia has been part of various states, which consequently led to changes and development of the institute of citizenship. It is inevitable to mention stateless persons. Stateless persons are persons without citizenship. The principles of acquiring citizenship are: ius sanguinis, ius soli and ius domicili. In practice, it is very difficult to apply only one principle. Because of that, they are often combined. The paper contains an overview of the principles through the Croatian Citizenship Act and acquisition in the world and Europe is also elaborated. Citizenship is often taken for granted, but citizenship opens up a whole range of questions and potential problems. Migrations also endangers citizenship for various reasons (endangerment of cultural and national identity, etc.). Although Croatia is a country of emigration, it still faces the problem of migrants. Globalization and the increasing movement of people imposes the need to adapt citizenship to modern requirements.