This paper puts forward the main elements of the theory of constitutional synthesis as a constitutional theory of European integration. Constitutional synthesis is both a political philosophy of European integration (which dilucidates what kind of polity the Union is an what is its basis of legitimacy) and a theoreticalframework capable of guiding constitutional adjudication in hard cases (such as the resolution of conflicts between European and national constitutional law). In essence, constitutional synthesis refers to aprocess in which already established constitutional states integrate through constitutional law, without losing their institutional structure and identity. We claim that there are three basic insights in constitutional synthesis. The first is that the constitutional law which frames and contributes to steer integration is characterised by the central role played by the constitutions of the participating states (by the regulatory ideal of a common constitutional law). The second is that the supranational legal order comes hand in hand with a supranational institutional structure which is only partially established at the founding, takes time to be rendered functional in aprocess where different national institutional cultures and structures try to leave their mark on the supranational level, and its structureis necessarily rendered more complicated as new institutions anddecision-making processes are added up to handle new policies. The third is that while supranational law is one, there are several institutions that apply the supranational law in an authoritative manner. In the last section of the paper, we complete the exposition of constitutional synthesis by considering how it is placed and how it relates to other political and legal theories of integration.