In order to achieve the objectives of integration policies, one of the crucial activities of the European Community was to ensure the approximation of laws of the Member States to the extent necessary for the functioning of the common/internal market. In this field, the adoption of the Lisbon Treaty has not brought along any significant changes. The substantive provisions conferring competence may be added, deleted or modified, as part of another common policy that contributes to the functioning of the internal market, without requiring changes in the procedural rules of the law approximation policy itself. Moreover, primary law includes several provisions on law approximation, without giving sufficient indication of any comprehensive definition of this concept, providing only a general and vague normative framework. For this reason, it was the judicial interpretation of the rules of cross-community policy of the approximation of legislation that came to complete the provisions of the treaties in a creative and evolutionary manner. In order to explain and analyse this evolution, this paper will first focus on the nature of the provisions on law approximation set by the treaties and then examine how those provisions were interpreted and completed by the Court of Justice.