Quorum sensing (QS), a cell-cell communication mechanism shown to influence bacterial behavior, allows bacteria to activate certain genes that will exhibit a particular phenotype when a bacterial species localized cell population reaches a certain threshold. Research focused on bacterial interspecies interactions and the role QS has on these interactions can contribute significantly to better understand how a particular species truly behave in the environment like the human microbiome. This dissertation work builds on this concept by studying how various streptococci interact with each other and determine the role QS plays in these interactions. A total of 26 unique streptococci isolates that consist of 19 species were used for this dissertation with a focus on the role the comCDE QS pathway has on any inhibitory interactions. Co-culture assays using these isolates revealed, along with genome mining, some interesting interspecies interactions. The opportunistic pathogenic streptococci, Streptococcus gallolyticus subsp. gallolyticus (Sgg), comCDE QS pathway and the phenotypes it regulates was studied in more detail which revealed that this QS pathway helps Sgg inhibit the growth of other streptococci through bacteriocin production. Structure-activity relationship studies on the Sgg comCDE QS peptide pheromone uncovered some interspecies QS interactions between Sgg and Streptococcus mutans QS peptides. Finally, this work identified a new strain of an emerging opportunistic pathogen, Streptococcus sinensis. Investigation into its comCDE QS pathway determined its QS signal and that it regulates competence. Global transcriptome analysis into S. sinensis competence regulon supports the current understanding of the Mitis group streptococci competence phenomena while also shedding light on subtle differences within this group.