In response to the very large number of quantitative indicators that have been put forward to measure the level of systemic risk since the start of the subprime crisis, the paper surveys the different indicators available in the economic and financial literature. It distinguishes between (i) indicators related to institutions, based either on market data or regulatory/accounting data; (ii) indicators addressing risks in financial markets and infrastructures; (iii) indicators measuring interconnections and network effects - where research is currently very active-; and (iv) comprehensive indicators. All these indicators are critically assessed and ways forward for a better understanding of systemic risk are suggested.