In this paper, we consider the role of system operators of various types (including ‘virtual’ system operators) in infrastructure in-dustries such as electricity, natural gas, railways and water supply. We focus primarily on some general issues about system operators (SOs) e what they are and what they do as well as their historical developmentoverthelast20years,beforeashortdiscussionoftheir functions and organisation in different industries. We outline the roles of alternative types of SO both in single-area jurisdictions e primarily nation states, except for water supply; and also in multi-area jurisdictions e.g. groupings of states or, for water supply, of regional water markets.1
Our focus is primarily on arrangements that have been agreed or are proposed for the European Union (EU). However, much of the development of SOs has been in the US. The development of ‘explicit’ SOs as separate institutions, i.e. independent system op-erators (ISOs), which control access to the network, started in US