The Mutual Assistance and Solidarity Clauses is the first comprehensive comparative analysis of two major innovations introduced by the Lisbon Treaty. Antonio-Maria Martino’s study focuses on the origins, scope, interpretation and technical aspects of both clauses, with a particular focus on the Solidarity Clause. He highlights the overlap between both provisions and the wide range of competences theoretically conferred upon the Union by the Solidarity Clause. The author also presents a critical outline of the negotiations with regard to the new Joint Proposal on the implementation of the Solidarity Clause by the Union, which will set out the framework for EU action in the event of a solidarity case.