Oral biofilms are complex structures formed on different surfaces of the oral cavity. They comprise a dynamic system where bacteria live within a multicellular community that protects them from external threats and allows bacteria to adapt and survive. Oral biofilms share general mechanisms of biofilm formation with other microbial communities in the rest of the human body: primary colonisers attach to a lightly coated substratum and, subsequently, these initial cells lead to the growth of microcolonies and maturation of the biofilm. The only oral environment that does not harbour a 'normal' oral biofilm is the root canal, but oral bacteria are able to enter this area in the event of a breach in the hard-tissue barriers. Current evidence shows that bacteria colonising the root canals of teeth form biofilm communities similar to their parent population (dental plaque). The presence of these biofilm communities in root canals of teeth has been related to inflammatory conditions of the dental pulp and the adjacent periapical tissues. Thus, a major goal of root canal treatment is the disinfection of infected root canals to their full extent. However, studies have proven that the disinfection of root canals is a real challenge due to it complex anatomy especially in the form of isthmuses, lateral canals and apical deltas. Schlagwörter: biofilm, complex anatomy, disinfection, root canal bacteria