Together with the Netherlands, the countries of Aruba, Curaçao and Sint Maarten form the Kingdom of the Netherlands. As they share the same Dutch citizenship, these four countries still also share the Dutch passport as the Kingdom of the Netherlands passport. As Aruba, Curaçao and Sint Maarten have small populations, the three countries had to limit immigration. To protect their population, they have the right to control the admission of people from the Netherlands. There is the supervision of the admission and expulsion of people from the Netherlands and the setting of general conditions for the admission and expulsion of aliens. Aruba is officially not a part of the European Union.
Although the equality of the countries in the Kingdom is explicitly laid down in the preamble to the Charter, which states "..considering that they have expressed freely their will to establish a new constitutional order in the Kingdom of the Netherlands, in which they will conduct their internal interests autonomously and their common interests on a basis of equality, and in which they will accord each other reciprocal assistance, have resolved by mutual consent", in practice, the Netherlands has considerably more power than the other constituent countries.