‘Biological Interfaces’ or ‘Biofilms’ are terms used to describe interfaces in natural and technical environments which are cultivated by microbial communities. Beside the interaction between the structure and function of biofilms, a strong coupling to the surrounding fluid also exists. Thus, we focus on the structural and functional characterization of biofilms from various origins such as nature and the lab, as well as full-scale reactors. An extended understanding of biofilm structure and function can be revealed by using a combination of imaging, spectroscopic, as well as micro-electrode and optode techniques. Moreover, multi-dimensional simulation approaches are used to gain a deeper understanding of related mass transport and transfer processes with respect to their impact on biofilm structure and function.