In the microbial world it is becoming apparent that many syntrophic, symbiotic and competitive interactions that occur within and between species are driven largely by a form of cell-to-cell communication known as quorum sensing, in which communication within and between species occurs through the use of highly specific signal molecules. In this work, evidence is presented through functional-genomics approaches that cell-to-cell signaling is a phenomenon not limited to mesophilic bacteria. In Thermotoga maritima, a hyperthermophilic bacterium, a previously uncharacterized small peptide (TM0504) that is very highly expressed under syntrophic growth conditions was found to have quorum sensing properties, inducing the cell-density dependent expression of glycosyltransferases to form exopolysaccharides. Exopolysaccharide production enabled the close association of T. maritima to the hyperthermophilic methanogen Methanococcus jannaschii, underlying the synthrophic transfer of hydrogen between species. Upon further examination, it was found that distinct life cycles exist within this syntrophic relationship, with rapid growth and aggregation in the co-culture followed by detachment of the two species in stationary phase. This process is postulated to be driven by an unknown quorum sensing system, allowing the detachment and spread of these organisms into new growth environments. In addition, evidence was provided that showed that Pyrococcus furiosus, a hyperthermophilic archaeon growing optimally near 100°C, both produces and responds to a recognizable form of AI-2, a furanosyl borate diester and known universal autoinducer of quorum sensing in mesophilic bacteria. As P. furiosus and all other members of the Archaea lack the LuxS enzyme involved in AI-2 biosynthesis in mesophilic bacteria, an alternative pathway must be involved. Purification of native AI-2 biosynthetic enzymes from P. furiosus crude cell extracts using a biological reporter assay allowed for the isolation of fractionated cell-free extracts that could convert adenosine to a species that triggered quorum sensing in a reporter strain of Vibrio harveyi. Through the use of the available genome sequence, it was proposed the production pathway for AI-2 involves the phosphorylation of ribose from adenosine through the activity of a eukaryotic-like MTA-phosphorylase (PF0016). In fact, the recombinantly produced MTA phosphorylase could complement fractionated P. furiosus biomass to produce enhanced levels of AI-2 activity from adenosine at 90°C. Other components of the pathway are under investigation, but likely includes a ribose phosphoisomerase (PF1258) to produce phosphorylated ribulose, which can be non-enzymatically converted to (4S)-4,5-dihydroxy-2,3-pentanedione (DPD). A potentially unique contribution of thermal energy in the conversion is proposed as this step is significantly accelerated at hyperthermophilic temperatures over rates observed at mesophilic temperatures, suggesting temperature may have had a role in directing the evolution of cell-to-cell signaling systems. Overall, these results suggest quorum sensing phenomena occurs in hyperthermophilic microorganisms, where it likely plays an important role in regulating intra- and inter-species interactions and defining microbial phenotypes.