This work reviews the basic theory of steady-state voltage instability; provides a physical interpretation of steady-state voltage instability; identifies some of the problems operators are presently experiencing; and gives simple procedures which can be used to determine system margin on a real-time basis. It is shown that steady-state voltage stability sensitivity investigations, using real-time data, can provide operators and system dispatchers with a viable procedure to estimate available system reserve margin to maintain adequate voltage security for a wide range of system parameters and operating conditions. The work also describes implementation of computer-based hardware for collecting real-time data which can be used to alert dispatchers of impending voltage problems and/or for conducting offline validation studies. >