Erosion is the most common cause of failures at earth-dams, dikes and levees, whether through overtopping and overflowing, or internal erosion and piping. This Chapter focuses on the concentrated leak erosion process. Concentrated leak erosion involves the formation of a crack or concentrated leak directly from the source for water to an exit point. Erosion initiates along the walls of the concentrated leak. This chapter provides the salient features of the Hole Erosion Test, and the latest breakthroughs in modeling the failure of an earth-dam or a levee by concentrated leak erosion and piping.