Many bacterial species use small chemical molecules as signaling factors. Signal molecules are released into the environment and once a threshold concentration level has been achieved, they trigger a response in the target cells. This system of cell-cell signaling, termed as ?quorum sensing?, plays an important role in the coordination of the growth and other processes in bacterial populations. Two types of signaling molecules have been identified: (1) short peptides, common to Gram-positive bacteria and (2) carboxy-acid derivatives commonly utilised by Gram-negative bacteria. Bacteria of the genus Streptomyces utilize butyrolactones as signaling molecules and thus form a distinct group among Gram-positives. This review focuses on two model regulatory systems based on carboxy-acid derivatives: the regulation of bioluminescence in Vibrio fisheri and the regulation of streptomycin biosynthesis in Streptomyces griseus.