This paper presents the main ideas and design concepts behind Modeller, a computer-aided modelling tool which aims to effectively assist in the develop ment of process models and to facilitate hierarchical modelling through a user friendly, graphical interface. The tool generates first principles based, mathematical models of (bio-)chemical processes in a quick and easy manner, based on a modelling methodology designed for such processes. Modeller is intended to be a component in a larger framework of modelling-assistant software tools and simulation packages. Its output can be transformed and interfaced with (or serve as input for) existing modelling languages and/or simulation packages (such as gProms, ASCEND, Omola, Modelica). Modeller has a graphical user interface, supports model reuse and is the result of an object oriented design.