The microbiological aetiology of periapical periodontitis of endodontic origin is generally accepted. Therefore, bacterial entry into the tooth both initially and secondarily to endodontic therapy is not desirable. Dental restorations and then root fillings in series are intended to prevent recurrent bacterial entry by blocking the access cavity and pulp canal system. This paper reviews whether this intent is realized practically, and what effect an incomplete seal might have on expressed recurrent disease. The aim is to communicate more recent research findings to a wider clinical audience.