This paper focuses on replication in DHT based structured overlay networks, and presents a theoretical framework for optimal replication in such networks with respect to query frequency. In particular, we determine the optimal replication factor for query-adaptive load balancing, and a topology specific placement strategy for an important class of DHT networks. We also provide a taxonomy of replication strategies in existing systems, discussing them with respect to this optimal scheme, and sketch further issues relevant for implementing this scheme in DHT networks. a.. keywords: Query-adaptivity, Optimal replication, Load-balancing, Distributed Hash Tables