The use of shield topology concepts to design interference control is described. Starting with the postulate that electromagnetic environments can he separated by closed shield surfaces, the proper design of essential compromises such as insulated power and signal conductors, and openings for access and ventilation are deduced. The role of grounding is described and the relation of grounding conductors to shield surfaces is deduced. Some guidelines are given for determining how effective the shield needs to be. It is concluded that the effectiveness of a shield is usually limited most by interference propagating on insulated conductors passing through the shield, followed by leakage through apertures and diffusion through the shield walls.