Abstract: As a new technology, wireless sensor networks attract considerable attention of academic researchers and people from industry. As the wireless sensor network nodes are usually smaller, and cheaper, and the networks and yet powerful and robust, they can be deployed in large number of applications ranging from habitat monitoring, medical applications, industrial automation to mission critical military applications. However, with the development of sensor networks and sensor network applications, the overall complexity of such systems is continuously increasing. Additionally, compared with traditional networks, wireless sensor networks have some unique characteristics. All of these make programming sensor networks non-trivial. Middleware supports programming abstract which facilitate the programmer task and bridge the gap between the application and the hardware. The characteristics of wireless sensor networks are described, and the problems which the development and design of wireless sensor network middleware layers must address are discussed. Then concrete middleware approaches are presented and discussed according to the programming paradigms used by them, and several classic approaches are compared by concentrating on how well they meet some criteria. Finally, some improvement directions are presented at several aspects of middleware design, such as communication paradigms, QoS support, security management, data processing, etc. The open research issues in this field are also pointed out.