Bacteria are known to regulate various gene expressions producing particular signal molecules termed "Auto-inducers" in the cell density-dependent manner. This physiological phenomenon termed "Quorum Sensing" has been discovered in many bacteria including normal bacterial flora and pathogens. Among Auto-inducers, Acyl-homoserine lactone has been best studied, which is leading with the human opportunistic pathogen, Pseudomonas aeruginosa. AHLs contain linear fatty-acyl side chains in the homoserine lactone moiety. Although the variation and specificity are determined by the sort of fatty acids, the number of fatty acid variations (approximately thirty) is absolutely less than that of AHL producers. However, new types of AHLs that have non-fatty acids in the side chain have been recently reported. This innovative finding enables to account for the diversity of AHL producing bacteria and also extend Quorum Sensing studies. In this paper, we will introduce novel classes of AHLs and their unique properties.