Interactions between and among bacterial species within a microcommunity occur in an environment that is distinct from the surrounding space. Interactions among species within one community are likely to be distinct from those within a different community. The context of mixed-species communities discussed in this chapter is primarily the human oral cavity, and the chapter focuses on the concept of distance-critical communication and its role in mediating commensalism as well as pathogenesis. The exhibition of extensive coaggregation partnerships by oral plaque bacteria suggests that distance-critical communication typically occurs within the tightly packed microcommunities known to characterize human dental plaque. Coaggregates formed among Methanothermobacter thermautotrophicus , butyrate-oxidizing Syntrophothermus lipocalidus , and acetate-oxidizing Thermacetogenium phaeum , indicating that coaggregations are relevant to interspecies hydrogen transfer among several syntrophic methanogenic consortia. Coaggregation and adhesion to host cells are primary characteristics of all oral fusobacteria. Fusobacteria coaggregate with all early and late oral colonizers. In place of producing strong toxins and enzymes, fusobacteria enhance their virulence functions through their ability to interact with other cell types. Many oral streptococci produce IgA1 proteases, which assist the community in evading the principal mediator of adaptive immunity.