This chapter outlines Material Flow Networks as a special modelling approach in the context of ecobalancing, that is, for modelling the environmental impacts of economic activities. This approach focuses on understanding the underlying material and energy transformations and the environmental impact of the resulting material and energy flows. The software tool Umberto® was the first product in the market for material flow analysis based on Material Flow Networks. This approach models the material and energy flows in production and distribution systems by means of Petri-Net notation with transitions (i.e., the material and energy transformation processes), places (i.e., inventories for material) and connecting arrows (i.e., energy and material flows). Using the Microsoft COM-technology, optimisation and discrete simulation models of economic processes can be embedded into ecologically oriented Material Flow Networks as complex external transitions. Three application examples (a transport optimisation model, a production and an inventory simulation model) are presented to demonstrate this useful approach of combining different modelling techniques in ecomanagement and its potential for ecoefficiency in resource and energy use.