This paper shows for the first time that distributed computing can be both reliable and efficient in an environment that is both highly dynamic and hostile. More specifically, we show how to maintain clusters of size O(log N), each containing more than two thirds of honest nodes with high probability, within a system whose size can vary polyno-mially with respect to its initial size. Furthermore, the communication cost induced by each node arrival or departure is polylogarithmic with respect to N, the maximal size of the system. Our clustering can be achieved despite the presence of a Byzantine adversary controlling a fraction τ ≤ 1/3 - e of the nodes, for some fixed constant e > 0, independent of N. So far, such a clustering could only be performed for systems whose size can vary constantly and it was not clear whether that was at all possible for polynomial variances. Copyright 2013 ACM.