Quorum sensing describes a bacterial communication system, which allows individuals to monitor the overall population density of the group. We study quorum sensing in proteobacteria. We are particularly interested in quorum sensing systems that make use of acyl-homoserine lactone signals. A major interest is in an opportunistic pathogen, Pseudomonas aeruginosa. Quorum sensing is important for virulence of this bacterium and quorum sensing is a target for therapeutic interventions. We use P. aeruginosa as a model to understand how quorum sensing is embedded in the complex regulatory networks of a cell, and we use it to study the evolution of cooperative behavior. We also study bacteria that make a newly discovered class of acyl-HSLs, the aryl-HSLs, and we study members of the genus Burkholderia with a particular focus on how quorum sensing confers a selective advantage to individual cooperators in a group.