The PREMIUM methodology is presented. This methodology provides a structured approach to the development of engineering line diagrams from process flowsheets. The generation of the method through a series of 'synthesis' and 'evaluation' steps is described. The structural features of the method-the different levels, groups, objectives and thought prompts are detailed. Associated 'support tools' for handling operating states and transitions and for providing detailed decision support are presented. The mode of application of the methodology and the outcome of its use in a live trial at ICI are outlined. The use of the methodology structure to support a number of features aimed at overcoming the shortfalls in current design practice is discussed. The shortfalls addressed are those associated with: safety, health, environment (SHE) and operability; the steady-state mindset; lack of formal training; lack of standardisation; poor management of information and poor management of interactions,