Horizontal transfer of genetic elements generates variations of prokaryotes and their genomes. Comparative studies of genomes revealed that it frequently occurred during archaeal and bacterial evolution. The book introduces a variety of studies related to horizontal gene transfer, gene silencing, plasmids, phages, transposons, and the emergence of microbes that degrade man-made xenobiotics and have antimicrobial resistance. Written by leading researchers in DNA traffic, the book is a valuable guide to horizontal transfer for both young scientists and experts in the field.