Like the principle of subsidiarity, the principle of proportionality regulates the exercise of powers by the European Union (EU). It seeks to set actions taken by EU institutions within specified bounds. Under this rule, the action of the EU must be limited to what is necessary to achieve the objectives of the Treaties. In other words, the content and form of the action must be in keeping with the aim pursued.
The principle of proportionality is laid down in Article 5 of the Treaty on European Union. The criteria for applying it are set out in the Protocol (No 2) on the application of the principles of subsidiarity and proportionality annexed to the Treaties.
Source: EUR Lex Glossary