Bacteria are capable “to sense” an increase of cell density population and to reply on it by the induction of special sets of genes. This type of the regulation named quorum sensing (QS) includes the production and excretion from cells into the medium low-molecular-weight signaling molecules (autoinducers, AI) which readily diffuse through a cell wall. As the bacterial population reaches a critical level of density, the concentration of these signaling molecules in medium increases as a function of density population. On reaching a critical threshold concentration, AIs bind to the specific receptor regulatory proteins which induce the expression of target genes. By means of AIs bacteria accomplish communication that is transmission of the information between bacteria belonging to the same or different species, genera, and even families — signaling molecules of some bacteria act on the receptors of others, resulting in coordinated reply of bacterial cells in population. Bacteria of different taxonomic groups use QS systems in the regulation of the broad range of physiological activities. These processes involve virulence, symbiosis, conjugation, biofilms formation, bioluminescence, synthesis of enzymes, antibiotic substances etc. Here we review the different QS systems of bacteria, the role of QS in bacteria communication and some applied aspects of QS regulation application.