Mutations are the raw material of evolution and play important roles in cancer, AIDS, and other human diseases. Because mutations are rare, they are hard to detect, particularly as they emerge. For most of the history of genetics, mutations were inferred by comparing the phenotypes of different individuals. More recently, genome sequencing has made it possible to discover all the mutations that separate two lineages, but this approach is expensive and has limited ability to resolve when mutations occur. We show that it is possible to directly see and count mutations as they emerge in live cells by exploiting the properties of the evolutionarily conserved DNA repair system Mismatch Repair (MMR). In 2010 we published a paper showing that non-repaired replication errors, i.e. emerging mutations, lead to the recruitment of several copies of the MMR protein MutL thus allowing us to detect the presence of such errors in live cells by fluorescent microscopy. The method that we developed allows, for the first time, detecting (i) genomic mutations independent of their phenotype, (ii) variability in mutations rates between different cells in population and (iii) mutagenesis in single cell during its life stages. We are currently investigating the fractions of the hypermutator cells in populations, i.e. cells with high mutation rates, during normal growth or upon stresses such as starvation, hypoxia, and drug treatments using microfluidics. This is relevant for evolution of antibiotic resistance and microbial pathogenesis strategies. These processes are driven by mutations and can be accelerated by increasing the fraction of cells with high mutation rates in populations.