This Dissertation studies the impacts of Demand Response (DR) programs on short-term reliability assessment of wind integrated power systems. A new algorithm including the effects of time, and components initial states is presented for short-term reliability evaluation. This algorithm involves a multi-segment optimal power flow approach to model the lead-time of DR and reserve resources. Taking into account the uncertainties associated with DR programs, a new model for these programs is also proposed. In addition, a reliability modeling is developed for wind energy conversion systems to be employed in short-term studies. The results show that DR programs can eliminate the negative impacts of wind energy volatility on power system reliability. It is also shown that the actual operating conditions, such as the uncertainties in demand-side participation and elasticity estimation, and the lead-time of remedial resources, may significantly affect the efficiency of DR programs. This Dissertation also provides a reliability perspective under the smart grid paradigm. Considering the advanced load control features of smart grid, this dissertation proposes the concept of “power systems with multi-level reliability”. Along with providing wider choices of electric supply services for consumer, such structure can relieve the social and economical consequences of load interruptions during contingencies. Keywords Demand Response, Short-term Reliability Assessment, Smart Grid, Uncertainty, Wind Integrated Power Systems