Abstract: Delay-tolerant networks (DTNs) are intermittently-connected networks that may suffer from frequent and long lasting disconnection due to various reasons such as mobility, power management, scheduling, etc. Representative delay tolerant networks include wireless sensor networks using scheduled intermittent connectivity, mobile ad hoc networks, satellite networks with periodic connectivity, village networks, wildlife tracking networks, and pocket switched networks, etc. Due to the broad application prospect, delay tolerant networks attract much attention. However, compared with traditional networks, there are no stable end-to-end delivery paths in delay-tolerant networks, and the routing problem is thus much more complicated. Most of the existing research work also focuses on this problem, and many delay-tolerant network routing algorithms are proposed. In this paper, the state of the art in these algorithms is summarized. At first, the evaluation criterion of performance is introduced. Secondly, different taxonomies for delay-tolerant network routing algorithms are presented. According to the routing strategy, all the algorithms can be categorized into the algorithms based on replication and the algorithms based on forwarding. According to the network model, the algorithms can be categorized into the algorithms based on active mobility model and the algorithms based on passive mobility model. Thirdly, the representative routing algorithms are described for each class of algorithms. Furthermore, the advantages and disadvantages of these algorithms are summarized. Finally the future directions of research in this area are discussed.