This position paper argues that the operational modelling approaches from the formal methods community can be applied fruitfully within the systems biology domain. The results can be complementary to the traditional mathematical descriptive modelling approaches used in systems biology. We discuss one example: a recent Petri net analysis of C. elegans vulval development. 1 Systems Biology Systems biology studies complex interactions in biological systems, with the aim to understand better the entirety of processes that happen in such a system, as well as to grasp the emergent properties of such a system as a whole. This can for instance be at the level of metabolic or interaction networks, signal transduction, genetic regulatory networks, multi-cellular development, or social behaviour of insects. The last decade has seen a rapid and successful development in the collaboration between biologists and computer scientists in the area of systems biology and bioinformatics. It has turned out that formal modelling and analysis techniques that have been developed for distributed computer systems, are applicable to biological systems