Extensible grammar-based rights expression system for processing rights expressions including an interpreter with plug-in subcomponents, a validator, and a framework. In another embodiment, system includes a framework having an extensible architecture with extensibility points for adding extensions to the grammar, and an interpreter, the extensions defining semantics and syntax of new rights expressions. A method for processing rights expressions is also provided having the steps of registering plug-in components, making a programmatic call, finding and invoking appropriate plug-in components, evaluating the request against the grant, and returning an authorization result. In another embodiment, method includes the steps of providing an extensible grammar-based rights expression system having an extensible architecture with an interpreter, evaluating the request against the grant using the interpreter, and returning an authorization result. The method may include the step of adding new extensions to the rights expression system to allow processing of new rights expressions.