The reliability and availability of the combined-cycle thermal power plants depends on the perfect operation of all its systems (e.g. gas turbine, heat recovery steam generator, steam turbine and cooling system). The Heat Recovery Steam Generator (HRSG) is the link between the gas turbine and the steam turbine process having the function of converting the exhaust gas energy of the gas turbine into steam. In the cooling water system, heat removed from the steam turbine exhaust in the condenser is carried by the circulating water to the cooling tower, which rejects the heat to the atmosphere. This chapter presents reliability and availability analysis of a 500 MW combined-cycle thermal power station aiming at defining the most critical components of the main pieces of equipment as for power plant availability. The cooling tower cells are detailed evaluated in order to improve there availability through risk analysis and reliability centered maintenance concepts.