Quorum sensing (QS) describes intercellular bacterial communication mechanisms which regulate bacterial gene expression according to population cell density by extracellular molecular signals. Quorum sensing represents central component of bacterial communication. ----- Quorum, by its definition, means minimal number of present members of community needed for accepting decision. Bacterial quorum (quorum sensing) means a minimal critical density of bacterial cells in bacterial population needed for common genetic expression and coordinated action during microenvironmental exchanges. There are four QS systems: Autoinducer type 1 (LuxR-I type), Autoinducer type 2 (LuxS type), Autoinducer type 3 (epinephrine/norepinephrine sensor signaling system), and Short peptide signaling system in Gram-positive bacteria. One type of bacteria can communicate with other (same type) bacteria and with other types of bacteria. Also, bacteria can communicate with fungi. ----- Bacteria use QS apparatus to coordinate gene expression, and to inhibit, activate or modulate transcriptional programs (found in eukaryotic epithelial cells and immune effector cells) among bacterial strains to control bacterial growth and metabolism, virulence expression and many other essential functions in bacterial populations. QS plays a role in the generation of biofilms, and microenvironment of the biofilm allows QS-phenomenology. Biofilms can be defined as an organized group of homogenous or heterogeneous microorganisms living within a self-produced matrix of polymeric substances which gets attached to the biotic or abiotic surfaces. Examples of biotic surfaces are respiratory and urinary mucosal surfaces, mucosa of oral cavity, surface of the tooth, and surface of chronic wound. Abiotic surfaces are intravenous and urinary catheters, prosthetic heart valves, intrauterine devices, contact lense surfaces, and pacemaker. ----- Biofilm is important in the etiopathogenesis of dental caries, sinusitis, periodontitis, and chronic otitis media, and chronic urinary tract infections, chronic wound infections, osteomyelitis, and chronic respiratory infections in patients suffering from cystic fibrosis. Understanding etiopathogenesis of bacterial communication mechanisms reveals new possible therapeutic approach by targeting quorum sensing with quorum sensing inhibitors and biofilm generation inhibitors.