Blackouts have a pronounced negative impact on the economy and on society. However, within a market environment and for economic reasons, power systems are exposed to increasing stress because they are usually operated close their stability limits. Under these circumstances, the blackout risk may increase. Most existing electricity markets provide dispatch solutions that are based on economic grounds and, generally, do not explicitly consider security issues. As a result, during real-time operation, the system operator must ensure system security and implement control actions as necessary. This book devoted to the development of procedures that ensure power system security in the context of real-time operation. The proposed procedures are intended to assist system operators in guaranteeing an appropriate security level both optimally and economically. In particular, the proposed procedures are based on optimal power flow problems that include voltage, small-signal, and transient stability constraints.