Nowadays the most salient trend with network is the increase in data rates while there is significant effort in developing new protocols and services. However, the traditional network devices which are custom logic based or pure software based, could hardly satisfy both performance and flexibility requirements. To overcome this obstacle, the parallel and programmable network processors have been involved into processing path of routers (switch). Besides network processors which are built on ASIP technology and optimized for network applications, possess the characteristic of hardware and software solution simultaneously. Network processors extend the classic store-and-forward pattern to store-process-and-forward, which makes room for complex QoS control and payload processing. In this paper, the related research work is introduced from two aspects, system and application, and the system issues and challenges of network processors are analyzed. In the end, the future evolution of network processors and associate researches are also speculated.