The International Commission on Large Dams (ICOLD) Bulletin 164: Internal Erosion of Existing Dams, Levees and Dikes, and their Foundations, presents an improved understanding of the mechanics of internal erosion. It shows that internal erosion initiates when the hydraulic forces imposed by water flowing through soil fill or foundation materials exceed the ability of the soils to resist them. This normally occurs when hydraulic gradients and water level are high. It gives methods to establish when internal erosion will initiate in concentrated leaks, by backward erosion, by contact erosion and by suffusion. It also gives methods to assess whether erosion will be arrested by some kind of filtering action in filters or shoulder fills. The paper presents the major findings of the Bulletin and demonstrates its use by examining case histories to explain why internal erosion did or did not occur.