Phil has made 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 participated in the development and application of metagenomics, the genome-based characterisation of microbiomes, which has revolutionised our understanding of microbial ecology and evolution. He has made several discoveries in environmental and clinical microbiology sometimes overturning decades of misdirected culture-based studies. Phil has published over three hundred papers in molecular microbial ecology including six Science & nine Nature papers.