Quorum sensing (QS) is an evolutionarily conserved mechanism of cell-to-cell communication utilized by hundreds of different species of microbes. QS is mediated by small molecules and their corresponding receptors, many of which are similar in structure and function. Consequently, microbes have developed ways to detect signals produced by their prokaryotic and eukaryotic neighbors and alter their behavior accordingly. These interactions have been termed interspecies and interkingdom signaling, respectively. In most described cases, bacteria use this sensory information to either up-regulate their own virulence or down-regulate the virulence of their neighbor. In this review, we describe many of the known interspecies and interkingdom signaling events that are mediated by quorum signals, and discuss the impact these may have on the ability of microbes to interact with each other and with their hosts.