The Fox Project: Advanced Language Technology for Extensible Systems Robert Harper, Peter Lee, and Frank Pfenning It has been amply demonstrated in recent years that careful attention to the structure of systems software can lead to greater flexibility, reliability, and ease of implementation, without incurring an undue penalty in performance. It is our contention that advanced programming languages---particularly languages with a mathematically rigorous semantics, and featuring higher-order functions, polymorphic types, and a strong module system---are ideally suited to expressing such structure. Indeed, our previous research has shown that the use of an advanced programming language can have a fundamental effect on system design, leading naturally to system architectures that are highly modular, efficient, and allow re-use of code. We are thus working to demonstrate the viability and benefits of advanced languages for programming real-world systems, and in particular Active Networks. To achieve this, we have organized our research into the areas of language technology, safety infrastructure, compiler technology, and applications. This report describes the current plans for this effort, which we refer to as the Fox project.