Dynamic system configuration is the ability to modify and extend a system while it is running. The facility is a requirement in large distributed systems where it may not be possible or economic to stop the entire system to allow modification to part of its hardware or software. It is also useful during production of the system to aid incremental integration of component parts, and during operation to aid system evolution. The paper introduces a model of the configuration process which permits dynamic incremental modification and extension. Using this model we determine the properties required by languages and their execution environments to support dynamic configuration. CONIC, the distributed system which has been developed at Imperial College with the specific objective of supporting dynamic configuration, is described to illustrate the feasibility of the model.