This paper reviews the experiences of Bechtel Water Technology in the application of programmable logic controllers. The paper outlines how experience of industrial disputes followed by the introduction of EC Drinking Water Directives during the 1980's generated a requirement to build a substantial number of unmanned automated plants. This resulted in a substantial increase in the number and sophistication of process control solutions produced. It outlines the challenges encountered during the life cycle of these sophisticated solutions and continues by considering how, in addition to the control requirements other factors such as the investment regime, operational constraints, and manpower skill levels influence the design of PLC control systems and led to different solutions being proposed for the water and wastewater treatment plants. It identifies how by the use of dynamic modelling techniques the company has been able to focus the application of advanced technology only to the areas where it is specifically needed. The paper reviews the adoption of PLC systems with SCADA interfaces, in preference to DCS packages. (10 pages)