One of the methods for overcoming impasses in the EU's legal setup has been differentiation between MS. The paper will analyse the reasons behind the adopted forms of differentiation, focusing on legal preconditions. The main question is whether national constitutional concerns played a role in the adoption of any of these solutions. Where (quasi-)federal states adopt similar differentiation between their constituent parts (asymmetry), importance is given to reasons, including constitutional, for asymmetry, since they may play a role when granting some constituent parts an asymmetric position. If it is confirmed that there exists a correlation between national constitutional concerns and differentiation, there would exist a level of constitutional contingency of differentiation among MS. In that sense, differentiation could be explained as a tool for the realization of respect of (constitutional) diversity in the EU.