Approximation is the adoption of measures establishing minimum rules related to the constituent elements of offences and sanctions. It aims at overcoming legal differences and creating the common ground required for cooperation. This contribution is part of a PhD research centred around necessity and feasibility of approximation of offences and sanctions. The central research question in this contribution is whether and to what extent approximation of the constituent elements of criminal behaviour is necessary to ensure the smooth functioning of both police and judicial cooperation in criminal matters in the European Union. Hence, police and judicial cooperation on the one hand and approximation on the other hand, are the lead concepts in this contribution. The lack of coherence in European criminal policy with regard to those concepts will be demonstrated and a remedy will be suggested. In this introductory section, first, the concept of approximation will be introduced. Second, the authors’ vision with regard to the discussions found in literature will be elaborated on. Third, the choice of case studies to analyse the development of police and judicial cooperation will be underpinned.