Quorum sensing provides a mechanism for bacteria to monitor one another's presence and to modulate gene expression in response to changes in population density. Camilli and Bassler (p. 1113) review how the synchronous response of bacterial populations to small molecule autoinducers that is involved in quorum sensing confers social behavior to bacteria. Autoinducers are packaged in a variety of ways and have varying half-lives, depending on their roles. Autoinducer signals are integrated within each cell by second-messenger systems, probably by cdiGMP signaling.