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There has been a very rapid increase in the size of power systems, and it is still growing. This growth has made the present day systems very big and complex. The complexity is arising especially due to many Interconnections, different types of loads and larger generating units. Also there is uncertainty in load forecasting. With EHV interconnections the disturbances are felt even in geographically distant parts of the system, increasing the severity of the disturbances. These factors make the satisfactory operating of the system a more difficult task. Added to this, Society is becoming more dependent on electric power; and it is essential to make the power available to customers without any interruption and within certain limits of voltage and frequency variations. This means that there is a great need to ensure that a system is as economical, reliable and secure as possible . High reliability of the system is usually obtained by following the principles of bulk power supply planning. However, due to delay in commissioning of new plants and equipment, equipment outages, unexpected load demands and disturbances, the system may be forced to operate in or close to critical and emergency conditions. Hence, in addition to sophisticated planning and designing of the system, an effective operation and control of power systems is essential. The maintenance of a system in a reliable and secure mode is becoming an Important part of the Job of the operators. In the past, more emphasis was given to economic operation; but in view of the consequences of power failures, reliable and secure operation of the system is being given more importance. Security assessment deals with the tracking of the system and deciding whether the present state is secure or not. If the system's state is found to be insecure, it forewarns the operator to take possible remedial action or to be ready for the disturbance if it takes place. This is done before any disturbance (which may lead to failu- e or deterioration in the quality of supply) occurS. Thus security evaluation is essentially an on-line procedure. The paper will discuss the different operating states of a power system, system security and different approaches for transient security assessment. Recent research results applying the pattern recognition approach will be summarized and presented.