Complicated cascading disturbances do occur in power systems and cause extreme social and economic impacts. A new Emergency Response System (ERS) is developed to defend power systems against these severe situations. Compared with traditional System Protection Schemes (SPS), an ERS has the 4 significant advantages. (1) The remedial action designed by ERS is more adaptive to system configuration and operating conditions, since it conducts computation online and takes real-time operating conditions as input. (2) Its initiating event set is much larger than that in traditional SPS, because it uses Dynamic Decision Event Tree (DDET) technique and dynamically increases the number of initiating events to be analyzed. (3) It detects, by incorporating long-term simulation and protective relay modeling into the simulator, many power system failures that were 'hidden' during traditional SPS remedial action design process. (4) The optimal action identification process is adaptive to power market information---By communicating with power market database, the most up-to-date cost factor is used to determine the optimal action.;This work also generalized the basic remedial action design features in such a way so as to make what has heretofore been highly application-specific technology---the SPS action logic design---into an automated and intelligent decision process. This contribution is important, because it is the fundamental enabler for the ERS, making it effective in an emergency scenario where response must be very fast. This contribution is also important because it serves to encapsulate, in a formalized way, the SPS design process; as a result it will be useful to SPS designers in reflecting on and improving upon what they do.;A demonstration system for this generalized remedial action logic design process is developed for proof-of-concept. The results on a test power system show that this approaches is feasible and very effective. Out of 41 initiating events in the test system that would result in power system failure, only 3 can be detected thus prevented by traditional SPS. The rest would rely on the ERS for identifying effective remedial actions.