Many modern computing environments involve dynamic peer groups. Distributed simulation, multi-user games, conferencing and replicated servers are just a few examples. Given the openness of today's networks, communication among group members must be secure and, at the same time, e cient. This paper studies the problem of authenticated key agreement in dynamic peer groups with the emphasis on e cient and provably secure key authentication, key con rmation and integrity. It begins by considering 2-party authenticated key agreement and extends the results to Group Di e-Hellman key agreement. In the process, some new security properties (unique to groups) are discussed.