Complex systems scientists cannot by themselves perform experiments on complex socio-technical systems. The best they can do is to perform experiments alongside policy makers who are constantly engaged in experiments as the design and manage the systems the systems for which they are responsible. In this context the nature of prediction in the implementation of real systems is much more complicated than it is in traditional science. The goals identified by policymakers change through time, and this is usually managed through the design and management processes. The combination of policy and design is the opportunity – the only opportunity – for complex systems scientists to engage and to be allowed to be involved in in-vivo experiments in large socio-technical systems. In turn this opens up new methodological approaches and questions for the science of complex systems