Bacteria in their natural ecosystems preferentially grow as polysaccharide-encased biofilms attached to surfaces. Although quorum-sensing (QS) systems directing the 'biofilm phenotype' have been extensively described in Gram-negative bacteria, there is little understanding of the importance of these systems in Gram-positive biofilm formation. Streptococci are a diverse group of Gram-positive bacteria that colonize epithelial, mucosal and tooth surfaces of humans. In several streptococci, competence-stimulating peptide (CSP)-mediated QS has been connected with competence development for genetic transformation. Recent work, especially with bacteria that inhabit the biofilm of dental plaque, has linked CSP stimuli to other cell-density adaptations, such as biofilm formation.