Bacteria were for a long time believed to exist as individual cells that sought primarily to find nutrients and multiply. The discovery of intercellular communication among bacteria has led to the realization that bacteria are capable of coordinated activity that was once believed to be restricted to multicellular organisms. The capacity to behave collectively as a group has obvious advantages, for example, the ability to migrate to a more suitable environment/better nutrient supply and to adopt new modes of growth, such as sporulation or biofilm formation, which may afford protection from deleterious environments. The “language” used for this intercellular communication is based on small, self-generated signal molecules called autoinducers. Through the use of autoinducers, bacteria can regulate their behavior according to population density. The phenomenon of quorum sensing, or cell-to-cell communication, relies on the principle that when a single bacterium releases autoinducers (AIs) into the environment, their concentration is too low to be detected. However, when sufficient bacteria are present, autoinducer concentrations reach a threshold level that allows the bacteria to sense a critical cell mass and, in response, to activate or repress target genes. Most of the bacteria thus far identified that utilize quorum-sensing systems are associated in some way with plants or animals. The nature of these relationships can be either amicable, as characterized by symbiotic bacteria, or adversarial, as seen with pathogenic bacteria. There are numerous bacteria that have components of a quorum-sensing system for which the phenotype regulated remains an enigma. Similarly, there are bacteria known to regulate a specific phenotype via quorum sensing for which one or more of the regulatory components have thus far eluded identification. In this review we give examples of pathogenic relationships, focusing on organisms for which many of the facets of their quorum-sensing systems have been elucidated.