The book contains a collection of high-quality academic and expert contributions dealing with the central question of whether the Lisbon Treaty needs further revision. Due to the difficulties European Union actors have encountered in implementing the Lisbon Treaty's reform and the inadequacies of the current legal framework brought to light by post-Lisbon practice, the volume focuses on possible innovations and functional approaches to improve the Union's response to the challenges confronting it.In doing so, the volume first takes a horizontal approach to the Treaty' revision and considers some constitutional features showing the interaction between the EU and its Member States (namely, the parameters of constitutional developments, the allocation of competences, the principles of solidarity and loyal cooperation).