A hierarchical approach to the design of control system structures is proposed. The approach has significant advantages over existing practice. 1. The task of placing all control loops on a complete and detailed flowsheet is reduced to more manageable proportions. 2. Attention is concentrated on strategically important control loops rather than those of secondary importance such as level in holding tanks. 3. Decisions on process and control system structures are taken together, allowing the latter to influence the former in a manner not possible with the conventional approach.