Power system reliability assessment methods can be categorized either analytical or as Monte Carlo simulation. Analytical methods represent the power system by a mathematical model and assess the reliability performance indices from the model utilizing mathematical solutions. On the other hand, Monte Carlo simulation methods, compute the reliability indices by simulating the actual process and random behavior of the power system, and can include any system effects or system processes which may have to be approximated in analytical methods. This paper describes a Monte Carlo simulation tool, which has been developed by General Electric (GE) Company for the reliability analysis of multi-area generation systems with explicit recognition of those unit and system operating considerations, rules, and constraints that influence system reliability indices. The basic features, capabilities and applications of the Monte Carlo simulation model are illustrated using the Mid-Continent Area Power Pool (MAPP) interconnected system reserve capacity obligation (RCO) study in this paper.