One of the major concerns in the operation and management of an electricity supply industry is to ensure its security.In electricity markets, all generating companies and power purchasing companies bid for the most profitable transactions, which may lead to congestion of certain corridors of the transmission network. The transmission congestion may be caused by violation of line flow, voltage and stability limits and, thus, undermine the system security. Therefore, it is important to determine the “Available Transfer Capability (ATC)” of the transmission system. The ATC information is required to be continuously computed, updated and made available to the market participants through the web based systems. This thesis has mainly addressed the issue of fast and accurate calculation of static as well as dynamic ATC and its enhancement through use of Flexible AC Transmission System (FACTS) controllers. The concepts and the approaches developed to address these issues should be especially useful to professionals in power industry as well as researchers working in power area.