Quorum sensing (QS) is an important signaling mechanism used by bacteria to coordinate gene expression among individuals in the population and hence, commits the unicellular bacterium into multicellular lifestyle. In the past half of a century, QS studies have focused on understanding the QS components, the molecular mechanisms, and the QS-regulated phenotypes using single species populations. Synthetic model communities comprised of two to three species have been increasingly employed in the last 20 years to simulate QS processes in environments where highly complex communities prevail, and to understand the role of QS in social interactions. Building on the foundations laid by these studies and owing to the recent technological advancements, we have begun to appreciate the various QS phenomena that occur in situ in both natural and engineered microbial communities. In this chapter, we focus on reviewing seminal works on QS phenotypes/roles in multispecies communities, starting from simplified synthetic model communities to highly diverse microbial communities in natural and engineered ecosystems. We highlight the examples of QS-regulated phenotypes that have strong implications and/or applications in the health of coral reef ecosystems, in the biogeochemical processes of element cycling, as well as in water and wastewater treatment processes.