Network monitoring is a vital part of modern network infrastructure management. Existing techniques either present a restricted view of network behavior and state, or do not efficiently scale to higher network speeds and heavier monitoring workloads. We present a novel architecture for programmable packet-level network monitoring that addresses these shortcomings. Our approach allows users to customize the monitoring function at the lowest possible level of abstraction to suit a wide range of monitoring needs: we use operating system mechanisms that result in a programming environment providing a high degree of flexibility, retaining fine-grained control over security, and minimizing the associated performance overheads. We present an implementation of this architecture as well as a set of experimental applications.