Horizontal gene transfer driven by self-mobilizable genetic elements allows the acquisition of complex adaptive traits and their transmission to subsequent generations. Transfer speeds up evolutionary processes as exemplified by the acquisition of virulence traits in emerging infectious agents and by antibiotic resistance in many human pathogens. Transfer is also costly because the vectors of horizontal transfer compete within genomes, have their own mobile elements and are often deadly. As a result, genomes are repositories of multiple immune systems from hosts and from mobile elements that interact in complex ways to drive gene flow in communities. The combination of evolutionary genomics and sequence analysis is now opening up these processes to show how they bring into the genome a constant flux of novel genes that favor the establishment and the invention of novel functions.