In spite of the growing body of knowledge on the function and regulation of quorum sensing (QS) systems, their genes are not always correctly identified in public databases. This chapter aims to present a few key concepts that can be used to classify QS systems according to the chemical nature of the signal, the mechanism of secretion, and the way external signal concentration can drive bacterial gene expression. The keys to our approach are gene topologies, i.e. the position and orientation of QS genes with respect to each other. We apply these concepts to review a few known QS and QS-related systems.