Professor Hugenholtz is a microbiologist who has made landmark contributions in the field of culture-independent analysis of microorganisms. He discovered and characterised numerous previously unrecognised major bacterial and archaeal lineages each with greater evolutionary divergence than animals and plants combined. He has been instrumental in the development and application of metagenomics, the genome-based characterisation of microbiomes, which has revolutionised our understanding of microbial ecology and evolution. His contributions are well recognised in medical and health science because they have increased our knowledge and helped to raise public awareness of the human microbiome and its role in health and disease.