<p>Many biological bimetallic active sites exhibit unsymmetric coordination environments, allowing the two metal centres to perform distinct catalytic roles. By contrast, most synthetic homogeneous catalysts rely on symmetric ligand frameworks for ease of synthesis. This represents a considerable divergence between natural and synthetic catalysts, leaving the functional importance of unsymmetric arrangement largely underexplored. In this Review, we highlight biological examples of unsymmetric bimetallic centres and their roles in catalysis. We examine how the inherent lack of symmetry — achieved through ligand differentiation or heterobimetallic design — has been used to mimic nature’s bimetallic sites. We also discuss recent advances that show how an unsymmetric environment can lower the barriers of bond activation and outline current challenges in probing the function of unsymmetric features, along with strategies to overcome them.</p><p></p>