Homogeneous catalysis is one of the pillars of modern chemical synthesis because it enables the sustainable preparation of molecules that find applications in medicinal chemistry, agrochemistry, and materials science. However, many catalytic reactions use hazardous reagents, are unpractical on laboratory-scale or limited in scope. Moreover, while a relatively broad set of catalytic reactions are available to construct chemical bonds, methods to cleave those, which could find applications in biomass and waste valorization, are rare. The overall objective of this ERC Starting grant is to develop a conceptually new approach to catalysis which relies on the development of catalytic reversible reactions. In a first approach, shuttle catalysis reactions are targeted which offer the possibility to transfer, reversibly, toxic reagents such as HCN, CO or Cl2 between two stable chemical compounds, providing a safer approach to organic synthesis. This approach also offers new opportunities to use catalysis to either functionalize petroleum feedstocks or defunctionalize biomass-derived feedstocks. In an second approach, we also develop new single-bond metathesis reactions which allow for the reversible exchange of functional groups to enable previously unthinkable chemical transformations. Taken altogether, these new methods provide a conceptually novel entry into the safe and sustainable preparation of important organic molecules.