Although DNA needs to be accurately replicated from one generation to the next, evolution depends on variations provided by changes in genotype. DNA sequences change over generations through various mutations, DNA rearrangements, and gene transfers between species. Bacterial and archaeal genomes experience a rapid flux, sometimes shuffling large clusters of genes between members of different taxonomic kingdoms. All this inter- and intraspecies DNA traffic has led ecologists to expand the definition of the microbial genome to include DNA in the cell plus all the DNA out in the environment to which the microbe has potential access. This chapter examines transient and heritable DNA movements among species, the competing processes of mutagenesis and mutation repair, and how these processes remodel genomes to build biological diversity.