The co-decision procedure introduced by the Treaty on the European Union, was conceived as an extension of the cooperation procedure. However, while in the latter the Council can, acting unanimously, disregard the opinion of Parliament, in the co-decision procedure there is no such possibility: in the event of disagreement, a conciliation committee made up of representatives of the Council and of Parliament has to arrive at a text that is acceptable to the two institutions.

The co-decision procedure now puts these two institutions on an equal footing in the legislative roles. Under this procedure, the Council cannot adopt a common position if the process of conciliation with Parliament fails. If no agreement is reached, the legislative process is liable to be broken off.

Co-decision has become by far the most important procedure in legislative practice and today concerns the following areas:

• non-discrimination on grounds of nationality (Article 12)
• combating discrimination based on sex, racial or ethnic origin, religion or belief, disability, age or sexual orientation (Article 13(2))
• freedom of movement and residence (Article 18(2))
• free movement of workers (Article 40)
• social security for migrant workers (Article 42)
• right of establishment (Article 44(1), Article 46(2), Article 47(1) and (2))
• visas, asylum, immigration and other policies relating to the free movement of persons (Article 67(4) and (5))
• transport (Article 71(1), Article 80)
• the 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 (except recommendations) (Article 151(5))
• public health (Article 152(4))
• consumer protection (Article 153(4))
• trans-European networks (Article 156)
• industry (Article 157(3))
• economic and social cohesion(Article 159)
• the European Regional Development Fund (Article 162)
• research and technological development (Article 166(1), Article 172)
• vocational training (Article 150(4))
• the environment (Article 175(1) and (3))
• development cooperation (Article 179(1))
• political parties at the European level (Article 191)
• access to the institutions' documents (Article 255(2))
• fraud (Article 280)
• statistics (Article 285)
• establishing a supervisory body for data protection (Article 286)