Communicating mobile processes. Introducing occam-pi. This paper introduces occam-pi, an efficient and safe binding of key elements from Hoare’s CSP and Milner’s $pi$-calculus into a programming language of industrial strength. A brief overview of classical occam is presented, before focussing on the extensions providing data, channel and process mobility. Some implementation details are given, along with current benchmark results. Application techniques exploiting mobile processes for the direct modelling of large-scale natural systems are outlined, including the modelling of locality (so that free-ranging processes can locate each other). Run-time overheads are sufficiently low so that systems comprising millions of dynamically assembling and communicating processes are practical on modest processor resources. The ideas and technology will scale further to address larger systems of arbitrary complexity, distributed over multiple processors with no semantic discontinuity. Semantic design, comprehension and analysis are made possible through a natural structuring of systems into multiple levels of network and the compositionality of the underlying algebra.