The normative model of operator behaviour constitutes the framework for the description of typical operator errors (i.e. deviations from the norm) and their causes. The normative model of operator behaviour will be based on the decision under risk. By this the methods of technical risk assessment, economic decision theory and psychology can be utilised in the assessment of operator behaviour. This view on operator errors is basically a technical one. The notion of operator error does not imply who is to blame for it. The normative model is meant to serve as a basis for an analysis of operator errors by means of psychological experiments, and the formulation of guidelines for the design of man-machine communication and user interfaces.