Recently, conventional concepts concerning the internal structure of wastewater biofilms have been brought into question. Most existing biofilm kinetic models assume that biofilms can be modeled in a one-dimensional (perpendicular to the substratum) fashion. There is much evidence indicating that mature biofilms may adequately be considered as growing as a continuum, although heterogeneously stratified with depth, and thus can be modeled in this way. However, there is recent evidence indicating that at least some biofilms grow in a much more heterogeneous format, with a highly channelized structure. This paper reviews the research on biofilm structure and the implications for substrate and nutient mass transport into biofilms. It then discusses the impact of the various growth models on biodegradation kinetics.