The phyllosphere represents the largest and most unique microbial environment on Earth, hosting an incredibly diverse array of microorganisms including bacteria, fungi, viruses, cyanobacteria, actinobacteria, protozoa, and nematodes. These diverse, rich microbial communities are marked by functional characteristics unique to their hosts that affect the physiology of the host plant, as well as the efficiency of ecosystems. In the last few years, considerable advancements have been made in phyllosphere microbiology, such as biodiversity, composition and dynamics of microbial communities, and their functional relationships. It has been established that host plants can recognize and selectively recruit beneficial bacteria, leading to a distinct community of root-endophytic bacteria compared to those found in the rhizosphere. Thus, colonization of plant tissues is a sophisticated process that involves complicated signaling between microbes and the host plant. Plants release several chemical signals from their roots, regulate nutrient availability, and shape the abundance and diversity of endophytic bacteria. The quorum-sensing mechanism of endophytic bacteria is essential for colonization of plant roots, even in the absence of plant exudates. N-acyl-homoserine lactones are the most common quorum-sensing signals in gram-negative endophytes, whereas peptides are usually used by gram-positive endophytes as quorum-sensing signals. This chapter explores the signals exchanged between rhizosphere microbial communities and the phyllosphere, highlighting their roles as biological control agents and discussing the functional importance of different microbial communities in both the host plant and the surrounding environment. This knowledge is essential for developing strategies to model and manipulate beneficial microbial consortia for use as biological control agents.