1 In our view, the mass balance principle is an extremely practical approach. A key ad-vantage is that product formulations and production processes can be gradually adapted to incorporate resources from renewable or recycled sources, while the quality of the end products remains stable. Sustainable versions of technically high-performance and established products can be offered that previously relied on the use of 'virgin materials' from the petrochemical industry. Raw materials that are based entirely or to a large extent on renewable starting materials are still at the beginning and therefore harbour risks with regard to availability, consistency and ...