Apart from removing obstacles to intra-Community trade, a customs union includes the harmonisation of customs regulations on trade with non-member countries. The efforts aimed at implementing such regulations in the European Union take two solid forms. On the one hand, the Community has established, and manages, a Common Customs Tariff (CCT) ; on the other hand, Community rules fit into an international context, whose evolution they must follow. Thus, arrangements agreed previously in the context of GATT and, henceforth, of the World Trade Organisation must be transposed into Community law [see section 23.4].

Customs union requires more than just having a common customs tariff. This tariff must also be applied according to identical rules throughout all Member States. Failure to do this could result in different values attributed to goods for customs purposes or different rules on the release of goods for circulation according to the importing Member State. The Community Customs Code (CCC) , which groups together all the provisions of the Community's customs legislation, aims precisely at removing the risk of different interpretations of EU rules in trade between the Member States and third countries [Regulation 450/2008, see section 5.2.3]. The common customs legislation grouped together in the customs code is an attribute of the customs union.