While the the use of markets to make new spectrum allocation decisions is becoming increasingly common in policy circles, such initial allocations are only that—initial allocations. The ability of market mechanisms to ensure that resources remain in their most highly valued use requires not only that these initial allocations be properly conducted, but also that secondary markets be well functioning. And though this is well understood in principle, the practice of secondary spectrum markets has been less well studied. In this paper, we have made an initial effort to understand the underlying principles of such secondary markets and the evolving policies toward secondary spectrum markets. We have also taken first steps toward quantifying important dimensions of the secondary spectrum markets. With this initial effort in hand, it is perhaps possible to begin to fashion policies that better enable the growth and development of these markets, and, more generally, the use ability of wireless technologies that rely upon this spectrum.