With the genome sequencing of hundreds of bacterial isolates per day and a vast and growing number of metagenomic sequencing projects on gut microbiomes in healthy and diseased people all over the world, it becomes feasible to explore the microbial diversity in us not only at the level of genera and species, but at strains. As two different strains of a prokaryotic species might only share 40% of the genes and can also have vastly differ in single nucleotide polymorphism (SNP) many aspects of a proper understanding of the microbial communities we host in the gut might only be revealed at this high resolution level. This proposal aims (i) to develop a robust methodology to characterize the SNP and gene content landscape from metagenomic shotgun data (ii) to explore patterns of variation in the human population to stratify geographically, but also in subpopulations such as families and to understand spreading patterns and the evolution of microbial strains as well as iii) to work towards medical applications, for example by monitoring fecal microbiota transplantation (FMT) at strain resolution or monitoring particular strain of interest in the population.