Operating system research has endeavored to develop micro-kernels that provide modularity, reliability and se- curity improvements over conventional monolithic kernels. However, the resulting kernels have been slower, larger and more error-prone than desired. These efforts have also failed to provide sufficient application control of resource management required by sophisticated applications. This paper describes a caching model of operating sys- tem functionality as implemented in the Cache Kernel, the supervisor-mode component of the V++ operating system. The Cache Kernel caches operating system objects such as threads and address spaces just as conventional hardware caches memory data. User-mode application kernels han- dle the loading and writeback of these objects, implement- ing application-specific management policies and mecha- nisms. Experience with implementing the Cache Kernel and measurements of its performance on a multiprocessor suggest that the caching model can provide competitive per- formance with conventional monolithic operating systems, yet provides application-level control of system resources, better modularity, better scalability, smaller size and a basis for fault containment.