We consider the concept of opportunistic spectrum access (OSA) - whereby radios identify unused portions of licensed spectrum, and utilize that spectrum without adverse impact on the primary licensees. OSA allows both dramatically higher spectrum utilization and near-zero deployment time, with an obvious and significant impact on both civilian and military communications. We discuss two broad classes of challenges to OSA: spectrum agility, which involves wideband sensing, opportunity identification, coordination and use; and policy agility, which enables regulatory policies to be applied dynamically using machine understandable policies. Focusing on spectrum agility, we present an architecture based on an OSA adaptation layer. We describe protocols for OSA, including a hole information protocol, idle channel selection and use, and an access protocol for the coordination channel. We present a simulation study, discuss insights, and show that even a simple protocol for opportunistic spectrum allocation can provide an order-of-magnitude performance improvement in throughput over a legacy system. Copyright 2006 ACM.