The last decade showed the central role of the mobilome for the evolution of microbial life. Plasmids, chromids, phages, gene transfer agents and transposable elements are responsible for quick and long term adaptations in a changing environment. The presence of a global genetic network is illustrated by the rapid transmission of antibiotic resistance genes between clinical isolates, soil bacteria and marine microbes. However, the relevance of extrachromosomal elements (ECEs) was – due to the lack of complete genomes – overlooked in most comparative genome studies. Therefore, we establish complete bacterial genomes of highest quality to distinguish between the chromosome and ECEs. This analytical paradigm shift was the basis for the discovery that plasmids are crucial for the ecological success of important bacterial lineages such as the Roseobacter group. Our current investigations are inspired by the working hypothesis that the mobilome is the key driver of microbial evolution.