The subgingival environment is a unique ecological niche where host–microbe interactions continuously occur, ultimately determining the balance that characterizes periodontal health or the disequilibrium featured in periodontal diseases. Importantly, knowledge of the subgingival microbiome has expanded dramatically due to the extensive use of next-generation sequencing techniques in both periodontal health and diseases, making it necessary to summarize and contextualize evidence from these types of studies. Consequently, this chapter first explores the anatomical and histological characteristics of the subgingival area and provides an overview of the microbial communities and immune surveillance mechanisms unraveling in this space and during periodontal health. Then, focus is placed on better defining periodontal inflammation as a main driver of microbial dysbiosis during disease. The subgingival microbiome in gingivitis and periodontitis is further analyzed, focusing on the environmental changes triggered by periodontal inflammation, as inflammation and tissue destruction lead to dramatic environmental changes that are associated with distinct microbial shifts. The microbial transition from health to gingivitis is explored in detail, discussing evidence stemming from studies dedicated to natural and experimental gingivitis. Finally, the dynamics of subgingival microbial communities in periodontitis and their connection with inflammation are explored, acknowledging the bidirectional nature of this relationship and highlighting its importance in determining health and disease in this setting.