The coordinated participation of Demand Response (DR) allows the improvement of the economic efficiency of the Electricity Market. This increase in economic efficiency is due to the fact that DR reduces peak demand and price volatility. Additionally, in a scenario with high penetration of stochastic renewable sources, DR promises to be a better alternative than the use of polluting and costly reserves to balance the variability of renewable generation. Countries such as the United States have prescribed that DR resource owners can offer this service as a resource supply for greater market transparency. Therefore, shaping the demand to reduce the peak and smooth its variation will improve the efficiency in the operation of the power system, generating large savings. There is a general consensus in the world on the favorable impact of DR in generating efficiency and greater flexibility in the operation of electricity systems. The challenge is to design the appropriate mechanisms to put its implementation into practice so that the full potential of DR is effectively exploited. The research work presented here is the design of an optimization model to determine the optimal operation of a DR aggregator that manages a portfolio of DR resources based on bilateral contracts of different characteristics to participate in the electricity market in the Day Ahead (DA) and Real Time (RT) instances.