The outcome of root canal treatment is influenced by its quality and epidemiological studies have reported a high prevalence of persistent apical periodontitis in teeth with sub-optimal root canal fillings. One of the main causes of root canal treatment failure is the persistence of intraradicular infection in the root canal system, which may not be due to a particular microorganism, but on the capability of communities of bacteria to adapt to changes occurring in the root canal environment following root canal treatment. Intraradicular infection can persist in the complex apical root canal anatomy such as canal irregularities, deltas and isthmus areas, and accessory canals. Bacteria in these untouched locations are primarily organised as biofilms - protected adhesive structures composed of microorganisms embedded in a hydrated polymeric complex. Numerous studies have demonstrated the presence of biofilms in root canal-treated teeth with persistent apical periodontitis. Microorganisms display an increased tolerance to antimicrobial agents and an enhanced pathogenic potential when growing as a biofilm. The biofilm mode of growth may constitute an important mechanism for the persistence of bacteria in the harsh environment of root canal-filled teeth. This review supports the argument for the inclusion of the biofilm concept in the aetiology of root canal treatment failure. Endodontic treatment should be provided to the highest standard if it is to tackle the intraradicular biofilm and reduce the risk of treatment failure. Keywords: biofilm, endodontic treatment failure, intraradicular infection, persistent apical periodontitis