The apportionment of seats within the European Parliament to each member state of the European Union is set out by the EU treaties. According to European Union treaties, the distribution of seats is "degressively proportional" to the population of the member states, with negotiations and agreements between member states playing a role. Thus the allocation of seats is not strictly proportional to the size of a state's population, nor does it reflect any other automatically triggered or fixed mathematical formula. The process can be compared to the composition of the electoral college used to elect the President of the United States of America in that, pro rata, the smaller state received more places in the electoral college than the more populous states.

The European Union (EU) is a political and economic union of 27 member states that are party to the EU's founding treaties, and thereby subject to the privileges and obligations of membership. They have agreed by the treaties to share their own sovereignty through the institutions of the European Union in certain aspects of government. State governments must agree unanimously in the Council for the union to adopt some policies; for others, collective decisions are made by qualified majority voting. These obligations and sharing of sovereignty within the EU make it unique among international organisations, as it has established its own legal order which by the provisions of the founding treaties is both legally binding and supreme on all the member states. A founding principle of the union is subsidiarity, meaning that decisions are taken collectively if and only if they cannot realistically be taken individually.