It has become increasingly difficult to unite all member states behind policies that some of them consider desirable or necessary to further EU integration. Differentiated integration may be a solution for this, but also may complicate decision-making. This by raising national constitutional obstacles that have to be cleared in addition to those encountered in the ‘ordinary’ or at least the ‘most obvious’ decision-making procedure, which is, depending on the subject matter, decision-making by all member states inside the EU legal order, or the conclusion of an EU (Amendment) Treaty. There are only very few additional national constitutional obstacles to differentiated integration inside the EU legal order. As regards differentiated integration outside the EU legal order, via treaties, two categories should be distinguished. The first consists of treaties which regulate subject matter which could have been regulated in secondary EU law. If we compare the national process of treaty conclusion to the EU decision-making process, the additional national constitutional obstacles are manifold. They differ from one-member state to another and may range from approval by parliament or by referendum, via constitutional amendments, to ratification. The second subcategory consists of treaties which regulate subject matter for which no competence exists at the EU level. Instead of transferring the lacking competence to the EU by way of an EU (Amendment) Treaty, member states can opt for an ordinary treaty. If we compare the national constitutional obstacles faced by these to similar obstacles faced by EU (Amendment) Treaties, no additional national constitutional obstacles have been detected.