In this paper we propose a framework for the management of the complexity involved in building material flow system models. The object-orientated paradigm is applied as a tool to break down the material flow system model, as well as the design environment, into modules. The designer is provided with a generic material flow system model from which to start with the design process. The model is built, evaluated and improved using a collection of software applications that have been developed to address partial problems in the material flow system design process. The organization of design data and design tools into logical units clearly establishes precedence between software and data. This feature opens up possibilities for computer-assisted design tool sequencing.