Sting is an operating system designed to serve as a highly efficient substrate for modern programming languages. It is designed to run on both parallel and distributed systems. This paper describes one of the important contributions of Sting - customizable policy management at both the kernel and user level of the operating system. Two well defined interfaces separate control issues from policy issues. These interfaces allow different, customized policy management modules to be implemented without changing the rest of the system. Customizable policy management makes Sting suitable for many different operating environments including real time, interactive, and computationally intensive. It also allows the user to choose or implement a policy management strategy that is best suited to a particular program.