In the context of (composite) geomembrane being widely used in many earthrock dams in China and abroad, the reasons for defects and defect classification in geomembrane on the dam surface or inside dams are discussed. Based on a summary of research on the defects and leakage of (composite) geomembrane in China and abroad, it is shown that the existing results cannot be directly used to solve the defect and leakage problems of composite geomembrane used in high earthrock dams. It is also shown that defect evolution, the influence of defectinduced leakage on the dam seepage field distribution and dam safety, and effective measures for leakage control are the development bottlenecks for composite geomembrane application in high earthrock dams. Therefore, laboratory tests should be performed to study the defectinduced leakage of composite geomembrane. It is also necessary to conduct systematic research on some issues related to defect evolution and defectinduced leakage in composite geomembrane used in earthrock dams based on theoretical analysis, numerical simulation, and feedback analysis of practical projects, so as to provide guides for leakage prevention and estoration of composite geomembrane as well as safety evaluation of high earthrock dams with composite geomembrane used for seepage control.