Subsidiarity is an organizing principle that matters ought to be handled by the smallest, lowest or least centralized competent authority. The Oxford English Dictionary defines subsidiarity as the idea that a central authority should have a subsidiary function, performing only those tasks which cannot be performed effectively at a more immediate or local level. The concept is applicable in the fields of government, political science, cybernetics, management, military (Mission Command) and, metaphorically, in the distribution of software module responsibilities in object-oriented programming (according to the Information expert design guideline). Subsidiarity is, ideally or in principle, one of the features of federalism.

The word subsidiarity is derived from the Latin word subsidiarius and has its origins in Catholic social teaching. The concept or principle is found in several constitutions around the world (see for example the Tenth Amendment to the United States Constitution^{[citation needed]}).

It is presently best known as a fundamental principle of European Union law. According to this principle, the EU may only act (i.e. make laws) where action of individual countries is insufficient. The principle was established in the 1992 Treaty of Maastricht, and was contained within the failed Treaty establishing a constitution for Europe. However, at the local level it was already a key element of the European Charter of Local Self-Government, an instrument of the Council of Europe promulgated in 1985 (see Article 4, Paragraph 3 of the Charter)^{[citation need}