This paper considers primary-secondary spectrum sharing when the primary is a cellular system. Sharing is allowed if primary communications can withstand additional interference, rather than if there are no primary communications. We consider cases when primary and secondary systems cooperate (cooperative sharing) and when they do not (coexistent sharing). For coexistent sharing, we have devised novel mechanisms that allow secondaries to determine whether they can transmit and at what power without causing harmful interference to the primary by querying a sensor network rather than communicating with the primary system. A sensor network observes a primary’s downstream communications to estimate upstream communications, and thus when additional interference would be tolerable. We show that even in highly utilized spectrum like a cellular band, a significant amount of communications among secondaries is possible. Moreover, if primary power consumption is a consideration, coexistent sharing is roughly as effective as cooperative sharing.