About ten years ago a mathematical model was presented which describes the spatial distribution and development in time of microbial species in mixed-culture biofilms. The model was based on the continuum approach and was one-dimensional in space. These two concepts still are the basis of practically all biofilm models used today. On the experimental side some remarkable new findings have been made in the past years: transport of dissolved components in the biofilm is not always due to molecular diffusion only, transport of particulate components can not be exclusively related to the net growth rates of the microbial species in the biofilm, the liquid phase volume fraction (porosity) in the biofilm is not a constant, and simultaneous attachment and detachment of cells and particles at the biofilm surface is an essential process. These experimental findings had a significant impact on our notion of biofilm systems and called for the integration of new processes in the original mixed-culture biofilm model. The new processes can reproduce most of the experimental observations, however, they are described by empirical mathematical functions. Their mechanisms and significance for biofilm behavior have not been completely elucidated yet. Thus, the extended mixed-culture biofilm model represents primarily a tool for research on biofilm processes.