Monitoring vulnerabilities has the potential to be an essential part of power system asset management both for network companies and regulatory authorities. Vulnerability can be defined as an internal attribute of the electric power system regarding susceptibility and coping capacity towards a certain threat. Previous studies have revealed that there is a need for new knowledge on vulnerabilities in the power system including interdependencies with other infrastructures. This paper gives an overview of the state of the art in literature regarding indicators to measure the vulnerability of an infrastructure and more specifically of the power system related to extraordinary events. The needs for vulnerability indicators are identified and discussed, and a common framework and basis of essential terms for vulnerability indicators is proposed. Furthermore, a multi-step approach for developing vulnerability indicators is presented. Examples are given of possible indicators for the power system.