This dissertation shows that operating systems can provide fundamental services an order of magnitude more efficiently than traditional implementations. It describes the implementation of a new operating system kernel, Synthesis, that achieves this level of performance. The Synthesis kernel combines several new techniques to provide high performance without sacrificing the expressive power or security of the system. The new ideas include: ffl Run-time code synthesis--- a systematic way of creating executable machine code at runtime to optimize frequently-used kernel routines--- queues, buffers, context switchers, interrupt handlers, and system call dispatchers--- for specific situations, greatly reducing their execution time. ffl Fine-grain scheduling--- a new process-scheduling technique based on the idea of feedback that performs frequent scheduling actions and policy adjustments (at submillisecond intervals) resulting in an adaptive, self-tuning system that can support real-time data streams.