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The microkernel story is full of good ideas and blind alleys. The story began with enthusiasm about he promised dramatic increase in flexibility, safety and modularity. But over the years, enthusiasm changed to disappointment because the first generation processor microkernels were insufficient and inflexible. Traditionally, the word kernel denotes the mandatory part of the operating system common to all other software. The kernel can use all features of a processor (e.g., programming the memory management unit); software running in user mode cannot execute such safety critical operations. Most early operating systems were implemented by means of large monolithic kernels. Loosely speaking, the complete operating system-scheduling, file system, networking, device drivers, memory management, paging, and more-was packed into a single kernel. In contrast, the microkernel approach involves minimizing the kernel and implementing servers outside the kernel. Ideally, the kernel implements only address spaces, interprocess communication and basic scheduling. INSETS: Frequently Asked Questions on External Pagers;Frequently Asked Questions on User-Level Device Drivers;Frequently Asked Questions on Memory Servers.