Bacteria are able to sense an increase in cell population density and to respond to it by the induction of a particular set of genes. This mechanism, called quorum sensing, includes in gram-negative bacteria the production and secretion of an acyl homoserine lactone, which diffuses through the cell wall, from the cell to the medium. Bacteria use the quorum sensing mechanism to regulate a variety of phenotype, such as bioluminescence, virulence factor production, biofilm formation, and motility, which are essential for the successful establishment of a symbiotic or pathogenic relationship with a eukaryotic host. Understanding of mechanism of quorum sensing creates the possibility of elaborating new drugs efficient against pathogens. It also suggests ideal targets for agricultural application of bacteria. In the review the basic concepts concerning the bacterial quorum sensing mechanism were discussed. In this paper some aspects of eukaryote-bacterial cross talk in natural environment were also described.