The increasing installation of renewable energygeneration requires the integration of new automation andcommunication components into a (smart) power grid. This, inturn, requires rigorous testing of these components and theirinteraction in order to guarantee safe, reliable energy supply.Co-simulation will become more and more important as meansof testing since smart grids are multi-domain systems and assuch too complex for one expert to model and analyze. How-ever, simulation always carries uncertainty, which might defeatthe purpose of testing safety-critical systems if not consideredproperly. The quantiﬁcation of this uncertainty is of paramountimportance for the design of reliable testing frameworks. Sincethis issue has not yet gained much attention in the smartgrid simulation community, this paper presents an exemplaryuncertainty quantiﬁcation of a grid simulation to underlinethe acuteness of the issue. The groundwork of an uncertaintyanalysis framework for co-simulation is presented and furtherrequirements of such a framework are discussed.