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Ensuring secure operation has always been of paramount importance in the safe and economic operation of power systems. However, the evolution of the electric power industry toward open markets has introduced a number of factors that have heightened the need for more rigorous security assessment and, in many cases, made it much more challenging. Some of these factors include: 1) aging transmission infrastructures→potential component failures; 2) lack of new transmission facilities→bottlenecks; 3) large numbers of small and distributed generators→uncertainty in dispatch patterns; 4) market-driven transactions→unpredictable transactions and system usage; 5) cutbacks in system maintenance→potential for failures such as flashovers to trees; 6) dependence on communications and computer systems→software/hardware failures may leave operators blind; 7) neglect of integrated system planning→insufficient/improper generation/transmission resources; 8) trend toward interconnection→exposure to cascading disturbances. These factors have (a) lead to greater uncertainty in day-today system operations, (b) increased the number of potential sources for system disturbances and (c) increased exposure to phenomena such as voltage instability and inter-area oscillations. As a result, in many power systems, it has become impossible to operate systems with an acceptable degree of security by using traditional operational planning studies that are conducted off-line and use forecast conditions to predict system security limits. This has been illustrated by the major system disturbances that have recently occurred in different systems around the world; highlighting the importance of being able to monitor and ensure system security. Therefore, the use of on-line security assessment is quickly becoming a necessity. This paper presents an overview of the critical components required for on-line security assessment and provides a description of an actual implementation of an on-line security assessment system.