This paper reviews the factors that influence the electrical stress required for initiation of electrical trees in crosslinked polyethylene (XLPE) insulation. Mathematical model relating the average tree inception stress with the defect tip radius of curvature has been provided for AC and impulse voltages. The resulting equation for AC treeing inception stress is verified by experimental measurements. Moreover, the effects of voltage magnitude, defect tip radius, and time of voltage application on partial discharge (PD) properties are studied, and the conditions for the initiation of bush- and branch-type trees are identified. For each type of tree, the variation of PD level with tree length is investigated experimentally. The results show that a minimum stress is required below which treeing cannot initiate in XLPE insulation. Moreover, during the early phase of tree propagation, tree length follows a self-limiting growth model and in this phase, tree length and the maximum PD levels are correlated. Copyright © 2005 John Wiley & Sons, Ltd.