A broad overview of online power system security analysis is provided, with the intent of identifying areas needing additional research and development. Current approaches to state estimation are reviewed and areas needing improvement, such as external system modeling, are discussed. Online contingency selection has become practical, particularly for static security. Additional work is necessary to identify better indices of power system stress to be used in online screening filters for both static and dynamic security analysis. Use of optimal power flow schemes to recommend optimal preventive and corrective strategies is presented on a conceptual level and the need for further developments is discussed. The area of online transient and dynamic security analysis is examined. Methods are being investigated to speed up for online application existing analysis techniques currently used in an offline planning mode. Transient energy function (direct) methods, expert systems and neural networks, fast approximate contingency screens, and parallel processing may also have significant roles in this area.