Bacterial quorum sensing (QS) systems are cell density-dependent regulatory networks that coordinate bacterial behavioural changes from single cellular organisms at low cell densities to multicellular types when their population density reaches a threshold level. At this stage, bacteria produce and perceive small diffusible signal molecules, termed autoinducers in order to mediate gene expression. This often results in phenotypic shifts, like planktonic to biofilm or non-virulent to virulent. In this way, they regulate varied physiological processes by adjusting gene expression in concert with their population size. In this review we give a synopsis of QS mediated cell-cell communication in bacteria. The first part focuses on QS circuits of some Gram-negative and Gram-positive bacteria. Thereafter, attention is drawn on the recent applications of QS in development of synthetic biology modules, for studying the principles of pattern formation, engineering bi-directional communication system and building artificial communication networks. Further, the role of QS in solving the problem of biofouling is also discussed.