Continuous culture techniques have been applied successfully to the modelling of the microflora at various human sites, but especially the mouth and the colon. Complex microbial communities can be grown stably and reproducibly for prolonged periods under controlled environmental conditions. Key parameters can then be varied independently so that true cause-and-effect relationships can be established. The utility of some models has been increased by (a) the construction of diverse but defined inocula, (b) the use of growth media which reflect the main nutrients of the habitat, (c) the provision of surfaces for biofilm formation, and (d) the use of linked culture vessels (multi-stage systems) to permit spatial or temporal heterogeneity. Comparisons of the properties of such communities with clinical data have supported the validity of such model systems. Applications of these models have ranged from determining the relative significance of dietary versus endogenous nutrients on community balance and metabolism, to the effect of antimicrobial agents. These modelling studies have enabled significant insights to be made into the functions and properties of the resident human microflora.