The provision of facilities for system security or the implementation of security control is the distinguishing feature of modern power system control centres. After a rather slow beginning, the provision of secirity functions has now become virtually a standard of control centre design. The practical realization of known real-time analytical techniques for security functions has been made possible by the successful development of static state estimation methods for online application. The need for state estimation is examined and the experience of the electric power industry in the development and use of this function is discussed.