This paper presents a review of operating reserves used in electric power systems to balance load and generation under high generation of renewable energy. This study includes a review of: (1) concept of operating reserves, (2) methodologies used in scientific research to model the Unit Commitment problem and (3) methodologies used to incorporate the variability of wind generation into the numerical models. The purpose of this research is to compare the different methodologies and determine which one of them would be an adequate tool to assess operating reserves in a real electricity market. The main conclusion of this study is that the stochastic evaluation of operating reserves, by stochastic programming or evaluating several scenarios under a deterministic approach, is the most appropriate alternative because it increases the accuracy of the solutions. The downside of this option is the increased time of execution, so it's recommended to use a scenario reduction technique to improve its computational performance.