With the increasing popularity of wireless communication networks and mobile handheld devices, there is a pressing need to extend traditional distributed applications to mobile environments. In this paper, we propose a novel solution to support mobile clients in publish/subscribe systems. The key of our solution is the two-phase handover (2PH) protocol. In comparison with existing solutions, our solution can guarantee the exactly-once and ordered event delivery to mobile clients with very low cost, and does not require mobile clients to wait for a long time to get the undelivered events upon reconnection. Concurrent movement of multiple clients is considered and mechanisms are provided to resolve the possible conflicts during the handover processes.