Cell communication in bacteria occurs through a vernacular of small diffusible chemical signals that impact gene regulation during times of high cell density. This form of intercellular signaling, known as quorum sensing, optimizes the metabolic and behavioral activities of a community of bacteria for life in close quarters. Quorum sensing is best characterized as a means of communication within a bacterial species, whereas competitive or cooperative signaling can occur between groups of bacteria or between bacteria and the host. These systems are often integrated into complex, multilayered signal transduction networks that control numerous multicellular behaviors, including biofilm formation and other virulence traits. In addition, quorum signals, sensors, and signaling pathways are increasingly recognized as having biological properties that extend beyond cell communication. The deeper understanding of microbial cell communication promises to shed light on the complexities of the host-microbe relationship and may lead to novel therapeutic applications.