Overlay networks provide infrastructures for a large variety of Internet applications, like file sharing, online gaming, and multimedia streaming. However, these networks often face unexpected node failures and network disconnections, causing the overlay to be partitioned into several components, which may seriously affect the performance of the network. In this paper, we analyze the cause of overplay partitions and its impact on the efficiency of the system. After explaining the notion of half-life, a measurement used to describe the evolution of peers in overlay network, we propose a new construct called half-success as a measure of critical partition. Furthermore, we propose an iterative method for the detection of potential partitions and a proactive strategy for the prevention of such partitions. Simulation experiments show that our detection method can efficiently find almost all of the cut vertices at a low cost. In addition, we demonstrate that the proposed proactive scheme is more effective and much faster than reactive approaches. © 2007 IEEE.