According to the property-rights model of cognitive radio, the primary terminals own a given bandwidth and may decide to lease it for a fraction of time to secondary nodes in exchange for appropriate remuneration. In this talk, an implementation of spectrum leasing is proposed whereby remuneration takes place in the form of cooperative transmission. On the one hand, the primary link attempts to maximize its quality of service in terms of either rate or probability of outage, accounting for the possible contribution from cooperation. On the other hand, nodes in the secondary ad hoc network compete among themselves for transmission within the leased time-slot. At first, a centralized implementation is investigated and cast in the framework of Stackelberg games. Then, a decentralized and opportunistic variant is studied that is based on HARQ protocols and auctions. Analysis and numerical results show that spectrum leasing based on trading secondary spectrum access for cooperation is a promising framework for cognitive radio.