This report documents the program and the outcomes of Dagstuhl Seminar 16421 Universality of Proofs which took place October 16-21, 2016. The seminar was motivated by the fact that it is nowadays difficult to exchange proofs from one proof assistant to another one. Thus a formal proof cannot be considered as a universal proof, reusable in different contexts. The seminar aims at providing a comprehensive overview of the existing techniques for interoperability and going further into the development of a common objective and framework for proof developments that support the communication, reuse and interoperability of proofs. The seminar included participants coming from different fields of computer science such as logic, proof engineering, program verification, formal mathematics. It included overview talks, technical talks and breakout sessions. This report collects the abstracts of talks and summarizes the outcomes of the breakout sessions