Accelerator-driven systems (ADSs) have been proposed for a variety of purposes. Different fuel cycles have also been associated with these systems. An analysis is made based exclusively on simple but well-established physics facts. Requirements related to the subcriticality level, the reactivity evolution during irradiation, and the energy needed to feed the accelerator are examined, in particular with respect to thorium- or uranium-based fuel cycles. Moreover, the somewhat forgotten aspect of the spallation neutron source importance is underlined to set up guidelines to optimize specific ADS design. The potential role of ADS to burn long-lived wastes is also stressed.