Biofilm is defined as a community of bacteria intimately associated with each other and included within an exopolymer matrix: this biological unit exhibits its own properties, quite different in comparison with those showed by the single species in planktonic form. The oral cavity appears as an open ecosystem, with a dynamic balance between the entrance of microrganisms, colonisation modalities and host defences aimed to their removal: to avoid elimination, bacteria need to adhere to either hard dental surfaces or epithelial surfaces. The oral biofilm formation and development, and the inside selection of specific microrganisms have been correlated with the most common oral pathologies, such as dental caries, periodontal disease and peri-implantitis. Many of these bacteria are usual saprophytes of the oral environment, that, in particular situations, can overcome and express their virulence factors: to better understand the mechanisms of these pathologies it's necessary to know the complex interactions between all the bacterial species inside the biofilm and host tissues and responses. The present paper is a review of the most significant studies on the biofilm development modalities, their correlations with either health or illness of the oral cavity, the bacterial co-aggregation strategies and the biofilm response to antimicrobial agents.