Many network technologies aim to exploit the bandwidth of high-speed links, which now achieve data transfer rates up to several terabits per second. As packet interarrival times shrink to a few tens of nanoseconds, network systems must address a transmission-processing gap by providing extremely fast data paths as well as high-performance subsystems to implement such functions as protocol processing, memory management, and scheduling. Today, network processors are an important class of embedded processors, used all across the network systems space-from personal to local and wide area networks. Network processor architectures focus on exploiting parallelism to achieve high performance. They usually employ conventional architectural concepts to accelerate the processing required to switch packets between different protocol stacks. The architectures support the mechanisms that network protocols implement in a specific stack by providing efficient data paths and by executing many intelligent network or more homogeneous links - for example, a set of Ethernet links. Although network processors can also handle packets concurrently from different protocol stacks, we describe only single-stack processing here. However, the arguments and results extend to a multistack environment.