We present a per-flow packet sampling method that enables the real-time classification of high-speed network traffic. Our method, based upon the partial sampling of each flow (i.e., performing sampling at only early stages in each flow’s lifetime), provides a sufficient reduction in total traffic (e.g., a factor of five in packets, a factor of ten in bytes) as to allow practical implementations at one Gigabit/s, and, using limited hardware assistance, ten Gigabit/s.