The traditional UNIX security model is simple but inexpressive. Adding fine-grained access control improves the expressiveness, but often dramatically increases both the cost of system management and implementation complexity. In environments with a more complex management model, with delegation of some management functions to parties under varying degrees of trust, the base UNIX model and most natural extensions are inappropriate at best. Where multiple mutually untrusting parties are introduced, ‘‘inappropriate’’ rapidly transitions to ‘‘nightmarish’’, especially with regards to data integrity and privacy protection. The FreeBSD ‘‘Jail’’ facility provides the ability to partition the operating system environment, while maintaining the simplicity of the UNIX ‘‘root’’ model. In Jail, users with privilege find that the scope of their requests is limited to the jail, allowing system administrators to delegate management capabilities for each virtual machine environment. Creating virtual machines in this manner has many potential uses; the most popular thus far has been for providing virtual machine services in Internet Service Provider environments.