A learning path on computational modelling, integrated into the Dutch lower secondary physics curriculum has been developed and tested in school practice. The instructional materials that have been developed cover the first two years of this curriculum. In the learning path, modelling has been systematically combined with experimenting and doing measurements. The graphical version of Forrester’s system dynamics has been used as a modelling approach. In this approach, model equations are represented by a graphical diagram consisting of a structure of icons. Important results of this developmental research project are the design principles that have evolved in the course of this project. Consequences of integration of modelling into the physics curriculum are discussed. It is shown that modelling requires a higher level of conceptual mathematical understanding than usual in physics education, but it is also shown how this higher level of understanding can be achieved by the students. Reality-based interpretation of the graphical diagrams can conceal an incorrect understanding of diagram structures. As a result, students seemingly have no problems interpreting these diagrams until they are asked to construct a graphical model without assistance. In spite of this, at the end of the learning path, students who have followed it are able to build simple models without teacher assistance.