The presence and the transformations of organic fractions within biofilms are not usually included in biofilm models, mainly owing to the lack of knowledge of the important processes, the stoichiometry and the kinetics. A conceptual biofilm model taking into account microbial physiology with regard to production of extracellular polymers and their transformation within the biofilm is suggested. The suggested components in the biofilm are the cell biomass and the various extracellular polymeric substances (EPS), such as polysaccharides, protein, humic substances and nucleic acids. In addition, environmental factors that influence the microbial exopolymer production and composition are taken into account. The suggested conceptual model can be helpful in structuring the design and evaluation of biofilm experiments focusing on biofilm structure and properties.