This figure illustrates a very important concept namely: scales of observation. Here we see a schematic of a pulp and paper machine and the entire system is termed the “Macroscale or systems scale”. At this scale biofilm growth is very complicated and hard to study. Biofilms grow on a variety of different surfaces with complex geometries and flow conditions. If we isolate a single tube within the system and look at a cross section we see a uniform geometry with hydraulic conditions specific to pipe flow. This intermediate scale is called the “mesoscale” and biofilm growth at this scale is better suited to study using laboratory methods. The third scale show is called the microscale. Here we are focusing on a very small location at the interface of the biofilm, the substratum and the bulk fluid. Biofilm processes observed at the microscale are influenced only by the substratum and the mass transfer between the biofilm and bulk fluid.