N-Acylated　homoserine　lactones　(AHLs)　are　produced　by　Gram-negative　bacteria　as　communication　signals　and　are　frequently　studied　as　mediators　of　the　＂quorum　sensing＂　response　of　bacterial　communities.　Several　reports　have　recently　been　published　on　the　identification　of　AHLs　from　different　species　and　attempts　have　been　made　to　study　their　role　in　natural　habitats,　for　example　the　surface　of　plant　roots　in　the　rhizosphere.　In　this　article,　different　analytical　methods,　including　bacterial　biosensors　and　chromatographic　techniques,　are　reviewed.　A　concept　for　assignment　of　the　structures　of　AHLs　is　also　presented.　The　retention　behaviour　of　derivatives　of　AHLs　containing　β-keto　or　hydroxyl　groups　and/or　double　bonds　has　been　evaluated　in　relation　to　the　separation　behaviour　of　AHLs　with　saturated　and　unsubstituted　alkanoyl　chains.　Samples　have　also　been　analysed　by　high　resolution　mass　spectrometry　(Fourier-transform　ion-cyclotron-resonance　mass　spectrometry,　FTICR-MS),　nano　liquid　chromatography-electrospray　ionization　ion　trap　mass　spectrometry　(nano-LC-MS)　and　by　the　aid　of　a　biosensor.　The　results　obtained　from　ultra　performance　liquid　chromatography　(UPLC),　FTICR-MS,　nano-LC-MS,　and　bioassays　have　been　compared　to　attempt　structural　characterisation　of　AHL　without　chemical　synthesis　of　analytical　standards.　The　method　was　used　to　identify　the　major　AHL　compound　produced　by　the　rhizosphere　bacterium　Acidovorax　sp.　N35　as　N-(3-hydroxydecanoyl)homoserine　lactone.　©　Springer-Verlag　2007.