This paper describes a new way of thinking about demand-side resources to provide ancillary services to control the grid. It is shown that loads can be classified based on the frequency bandwidth of ancillary service that they can offer. If demand response from loads respects these frequency limitations, it is possible to obtain highly reliable ancillary service to the grid, while maintaining strict bounds on the quality of service (QoS) delivered by each load. It is argued that automated demand response is required for reliable control. Moreover, some intelligence is needed at demand response loads so that the aggregate will be reliable and controllable.