This report presents a framework and methodology to develop indicators to monitor the vulnerability of the electric power grid. It establishes a scientific foundation by defining vulnerability and the different dimensions threat, susceptibility, coping capacity and criticality. The process of indicator development is described and exemplified with cases studies from two grid operators and selected critical power lines in their supply area. Several example indicators are developed with focus was on the technical condition of the power lines. The theoretical framework and the practical experiences with the case studies showed that the development of leading indicators which can give information about the future vulnerability is still a remaining research challenge.