Microbial biofilms, where organisms are intimately associated with each other and a solid substratum through binding and inclusion within an exopolymer matrix, are widely distributed in nature and disease. In the mouth, multispecies biofilms are associated not only with dental plaque and tooth decay but also with soft tissues of the buccal cavity and with most forms of periodontal disease. Organization of micro-organisms within biofilms confers, on the component species, properties which are not evident with the individual species grown independently or as planktonic populations in liquid media. While many of these properties relate to the establishment of functional, mixed-species consortia within the exopolymeric matrices, others relate to the establishment of physico-chemical gradients, within the biofilm, that modify the metabolism of the component cells. A consequence of biofilm growth that has profound implications for their control in the environment and in medicine is a markedly enhanced resistance to chemical antimicrobial agents and antibiotics. Mechanisms associated with such resistance in biofilms will form the substance of the present review. While some aspects of biofilm resistance are yet only poorly understood, the dominant mechanisms are thought to be related to: (i) modified nutrient environments and suppression of growth rate within the biofilm; (ii) direct interactions between the exopolymer matrices, and their constituents, and antimicrobials, affecting diffusion and availability; and (iii) the development of biofilm/attachment-specific phenotypes.