Most microorganisms in nature live in multispecies communities attached to a substratum-biofilms. Within these communities, organismal interaction is spatiotemporally defined. Because biofilms exist at an interface, their environment is characterized by gradients of nutrients that encourage spatial and metabolic diversity within the population. Oral bacterial biofilms were among the first human-associated biofilms to have been extensively investigated. They are diverse in species, and that diversity reflects the range of habitats within the oral cavity. Oral bacterial communities can be studied in vitro and in vivo. These studies have yielded information on interorganismal interactions and the developmental patterns within the communities. The wealth of information on these communities, coupled with their accessibility in their natural state, firmly establishes them as paradigm systems in biofilm research.