The power system in many countries around world has been shifting from a regulated system to a deregulated and uncertain, competitive market environment. This exerts unprecedented and severe pressures on the existing transmission system by worsening flow congestion and decreasing security margins. On the other hand, maintaining system reliability is still the basis for the proper planning and operation of the bulk electric power system. The traditional deterministic security assessment approaches tend to be conservative by emphasizing the most severe, credible event. With the increased competition, engineers face more pressure, from economic imperatives in the market places, to operate the system closer to or even beyond the traditional deterministic limits. Therefore, more refined methods are needed to provide improved security assessment results, which should be able to take into account the probabilistic nature of many uncertain variables in the decision-making environment. Motivated by this need of the industry, this research is aimed at developing quantitative risk based methodologies for system security assessment with the consideration of the probabilistic nature of electric bulk system behavior. The work includes five parts: impact assessment for risk based security assessment, risk assessment for transformer overload, risk assessment for special protection systems, risk assessment for bilateral transactions, and risk based optimal power flow. Impact assessment for risk based security assessment provides a method to measure the cost consequence of different insecurity problems in the power system. Risk assessment for transformer overload, special protection systems and bilateral. transactions provide quantitative risk measurement which is helpful for the system operator to monitor the system security condition. Finally, the risk based optimal flow provides a tool for system operator in making decisions about balancing the benefits and costs in the current competitive markets.