Developing access control extensions for operating systems is an expensive and time-consuming task. Mechanisms for access control extension lag behind industry standard extension solutions for file systems, process schedulers, and device drivers, and suffer from a number of serious flaws in modern operating system kernels. The TrustedBSD MAC Framework provides a flexible and modular environment for access control extensions on the open source FreeBSD operating system, reducing the cost of implementation by abstracting out common elements found in many extensions. A variety of security policies have been implemented using the MAC Framework.