This thesis investigates the role of the mandatory requirements relating to the public interest as a limiting factor for the European legislator to design harmonisation measures, an issue which, while relevant to European legislation, has not yet been examined in detail. The mandatory requirements established in the Cassis de Dijon judgment ensure that Member States can warrant their national public welfare interests on their sovereign territory. The ECJ has consistently expanded the set of mandatory requirements. Concurrently, the European legislator has harmonised a multitude of areas within the European Union and in some instances has conclusively ruled over the protection of mandatory requirements (e.g. in the Services Directive) and in doing so has displaced the Member States from their position as protection guarantors. This thesis investigates to what extent the European legislator is authorized to do so and arranges the mandatory requirements into a practicable model solution.