Abstract: We present opportunistic renewal, a lease management protocol designed to keep distributed file systems or distributed shared memories consistent in the presence of a network partition or other computer failures. Our treatment includes an analytical model of the protocol that compares performance with existing lease protocols and quantifies improvements. In addition, this analytical model provides the structure to understand message overhead and availability trade-offs when selecting lease parameters. We include results demonstrating that opportunistic renewal substantially reduces the network overhead associated with lease renewal. As a corollary, opportunistic renewal can reduce the lease length at any given network overhead; e.g., by a factor of 50 at 1% network overhead. Lower overhead makes leasing less intrusive and shorter lease periods allow a system to recover from failure more quickly.