This issue is dedicated to the memory of Michel Blot, who died in a tragic accident in the French Alps in September 2002, bringing his fruitful scientific activities to a sudden end. After having completed his basic and doctoral education in population biology of eukaryotic organisms, Michel Blot joined my laboratory in 1989 with the intention of becoming familiar with microbial genetics and of combining the two fields of population biology and microbial genetics in studies on mechanisms of biological evolution. His contributions to this interdisciplinary approach rapidly attracted the attention of the scientific community, bringing him a well merited recognition of his scientific activities which he deployed for the last seven years as Professor and Group Leader at the University J. Fourier in Grenoble, France. The articles comprising this special issue have been written by colleagues and friends of Michel Blot and shall document the genetic plasticity of microbial genomes and its impact on microbial evolution. They complement related reports on bacteriophages, on integrons and on repetitive DNA sequences in microbial genomes published in recent special issues of this journal (Refs. [8–10]). The general relevance of studies on microbial genetics and microbial population biology for a deeper understanding of molecular evolution shall briefly be outlined in this editorial. The Neo-Darwinian theory of biological evolution was elaborated before it became known that genetic information is contained in DNA molecules. While this theory postulates that it is genetic variation which drives the evolutionary process, it cannot explain the molecular nature of genetic variation. With the advent of microbial genetics identifying DNA as the carrier of genetic information, and shortly thereafter, with the description of the doublehelical structure of DNA molecules, the door was open to investigating molecular mechanisms of genetic variation.