Quorum sensing (QS) is widespread in the microbial world, including pathogens, symbionts, and plant beneficial bacteria. With the help of diffusible autoinducer signal molecules, QS active bacteria change their lifestyle in the plant environment. N-acyl-homoserine lactones (AHLs) are the best studied QS autoinducers, and a broad spectrum of analysis methods is available including biosensors, chemical, and immunological techniques. This allows screening, identification, and in situ monitoring of virtually any type of AHL molecule. Not only do bacteria show a different expression profile when exposed to elevated levels of AHLs, but also the plant is responding with Ca2+ fluxes, NO production, followed by systemic responses like increase of salicylic acid content, priming of defense reactions, and morphological changes. Vice versa, so-called LuxR-solos in rhizosphere colonizing bacteria, i.e. bacteria with functional AHL receptors but without AHL synthase, are able to perceive AHL-like signaling compounds from the host plant. This indicates the development of a specific communication pathway between plant and bacteria. Thus, it becomes ever clearer that bacterial QS signaling is essential for the establishment of symbiotic and beneficial interactions of bacteria with their host plant.