Irrigation is regarded by many as being the most important part of root canal treatment. It has several different functions and goals depending on the type of the irrigant used: irrigation reduces friction between the instrument and dentin, improves the cutting effectiveness of the fi les, and dissolves organic and inorganic matter. It also cools the tooth and the fi le; it has a washing effect and removes loose debris and bacteria from the canal. Last but not least, irrigation acts against root canal biofi lms. Irrigation is also the only way to impact those areas of the root canal wall not touched by mechanical instrumentation. The factors that remain a challenge in the irrigation and disinfection of the root canal include biofi lm resistance, poor penetration of the irrigant, and exchange of irrigants in the highly complex root canal anatomy. Progress in the search for better irrigants and irrigant delivery is necessary. A variety of different study models have been used in endodontic research on irrigation. One of the issues is how to make a rational choice for a study model that is relevant for the question at hand. This article presents an overview of the methods and models that have been used in endodontic literature to study irrigation.