Until now, the used methods for assessment and allocation of the voltage control and reactive power support ancillary service, it are not solved the problem of ensuring sufficient reserve of dynamic reactive power to minimize the risk of voltage collapse. This thesis presents a novel methodology for the evaluation, assessment and allocation of the provision of this service, associating to the voltage stability margin index, the value of the reactive power required for voltage control and for minimizing the risk of voltage collapse. Also, this thesis propose and develops a new methodology that allows the participation of demand resources by associating the stability margin index with the opportunity cost of the demand resources inclusion in the provision of voltage control and reactive power support ancillary service