[en] The Kalundborg industrial symbiosis serves as an excellent example of inter-organizational collaboration, showcasing the principles of industrial ecology and the circular economy. Since the 1960s, local companies have developed a network of material and energy exchanges, turning waste and by-products into reusable resources in a mutually beneficial system. This model not only reduces waste and greenhouse gas emissions but also results in substantial savings in energy and raw material costs. Key participants in this symbiosis include companies such as Novo Nordisk, Novozymes, and Statoil, alongside the Asnaes power plant. These organizations exchange flows of heat, steam, gypsum, and biomass, creating an integrated network where one company's waste becomes a resource for another. The industrial symbiosis has significantly lowered CO2 emissions and saved millions of cubic meters of water, as well as tons of residual materials annually. This research note highlights that trust and inter-organizational collaboration are essential to the success of this circular economy model, while recognizing that its application in other regions will require adjustments tailored to local conditions.