Many plant-associated bacteria produce and utilize diffusible N-acyl-homoserine lactones (AHLs) to regulate the expression of specific bacterial genes and operons. AHL-mediated regulation utilizes two genes that encode proteins similar to the LuxI/LuxR system originally studied in the marine symbiont Vibrio fischeri. The LuxI-type proteins are AHL synthases that assemble the diffusible AHL signal. The LuxR-type proteins are AHL-responsive transcriptional regulatory proteins. LuxR proteins control the transcription of specific bacterial genes in response to the levels of AHL signal. To date, AHL-mediated gene regulation has been identified in a broad range of gram-negative bacteria, most of which are host-associated. However, it seems unlikely that such a widely conserved regulatory mechanism would be limited only to host-microbe interactions. These signals probably play central roles in ecological interactions among organisms in microbial communities by affecting communication among bacterial populations as well as between bacterial populations and their eukaryotic hosts.