The codecision procedure was introduced in the Treaty on European Union (TEU). In contrast to the cooperation procedure, in which the Council is not ultimately bound by the Parliament's opinion, the codecision procedure ensures that the Parliament's views are fed into the legislation.

If the Council and Parliament do not reach agreement, the matter is referred to a conciliation committee composed of representatives of both organs. If the conciliation procedure does not result in an agreement, the legislation cannot be enacted.

The European Parliament and the Council thus have equal legislative powers when the codecision procedure is applied. Most of the legislation adopted within the scope of the first pillar is subject to the codecision procedure.

The codecision procedure is by far the most important legislative procedure.

It is applicable in the following areas:

• discrimination on grounds of nationality (Article 12)
• discrimination based on gender, racial or ethnic origin, religion or belief, disability, age or sexual orientation (Article 13(2))
• right to move and reside freely within the territory of the Member States (Article 18(2))
• freedom of movement for workers (Article 40)
• social security for migrant workers (Article 42)
• freedom of establishment (Article 44(1), Article 46(2), Article 47(1) and (2))
• visas, asylum, immigration and other policies related to free movement of persons (Article 67(4) and (5))
• transport (Article 71(1), Article 80)
• internal market (Article 95)
• employment (Article 129)
• customs cooperation (Article 135)
• social policy (Article 137(2))
• equal opportunities and equal treatment (Article 141(3))
• implementing decisions relating to the European Social Fund (Article 148)
• education (Article 149(4))
• culture (with the exception of recommendations) (Article 151(5))
• health (Article 152(4))
• consumer protection (Article 153(4))
• trans-European networks (Article 156)
• industry (Article 157(3))
• economic and social cohesion (Article 159)
• European Regional Development Fund (Article 162)
• research and technological development (Article 166(1), Article 172)
• vocational training (Article 150(4))
• environmental protection (Article 175(1) and (3))
• development cooperation (Article 179(1))
• political parties at European level (Article 191)
• access to documents of the European institutions (Article 255(2))
• measures to counter fraud (Article 280)
• statistics (Article 285)
• establishment of an independent supervisory body for data protection (Article 286)