Chemical communication between cells ensures coordination of behavior. In prokaryotes, this chemical communication is usually referred to as quorum sensing, while eukaryotic cells signal through hormones. In the past years, a growing number of reports have shown that bacterial quorum sensing signals, called autoinducers, signal to eukaryotic cells, mimicking hormones. Conversely, host hormones can signal to bacterial cells through converging pathways to autoinducer signaling. This inter-kingdom signaling mediates symbiotic and pathogenic relationships between bacteria, mammalian and plant hosts.