To prevent differentiated integration from being itself a source of unfairness, the report identifies certain substantive and procedural conditions that it should meet to remain fair. Substantively, no member states should be excluded from a policy if they would become worse off than they currently are as a result, nor should a state be allowed to opt out of a policy or be exempted from meeting certain standards if that would render the participating or complying states worse off than they are. Procedurally, both exclusions and exemptions should be agreed either unanimously by representatives of all member states when negotiating the accession of new members or amending the Treaties, or result from a member state choosing not to participate in enhanced cooperation by at least nine member states. Political party actors express mixed views of differentiated integration. While some view it as a pragmatic way forward for the EU, others worry about its negative implications for political equality, solidarity, and unity. Party actors from poorer and less integrated member states are generally more sceptical about differentiated integration than actors from older and richer member states because they worry about being left behind. This project received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement number 822304. The content of this document represents only the views of the InDivEU consortium and is its sole responsibility. The European Commission does not accept any responsibility for use that may be made of the information it contains