There has been much recent interest in allowing commercial providers to utilize federal spectrum on a shared basis. In such scenarios, the use of secondary licenses has been proposed as a way to provide service providers with greater predictability about the service that can be offered and thus help encourage investment. However, since the spectrum is shared, such a license would differ from traditional exclusive use licenses in that the given spectrum may only be intermittently available. In this work we discuss several issues related to such intermittent spectrum and in particular consider different market structures for allocating such secondary licenses to different service providers. In each case we characterize the complexity of the underlying efficient allocation problem and discuss market mechanisms for implementing such an outcome.