The paper describes the application of an interactive identification procedure, based on recursive least-squares analysis and supported by a graphics-display facility. The derivation of a multivariable equation is reviewed. This is structured in terms of inputs and outputs, and is demonstrated to provide a very effective means for the modelling of the dynamics of industrial plant. Application is to a multiple-effect evaporator that is used as a first stage in the concentration of milk for the preparation of milk powder. The interactive facility is integrated into a scheme for the design and implementation of hierarchical computer-control systems for the milk-powder plant.