The dynamic spectrum-sharing technique (“cognitiveradio”), where secondary users opportunistically utilize temporarily or locally unused spectrum, has emerged as a prime candidate technology for relieving the perceived spectrum shortage in the lower frequency bands. Makinga realistic assessment of the amount of spectrum availablefor secondary services was the objective of the EU FP7QUASAR project. In the project, it was found to be fundamentally difficult to reliably determine which part of the spectrum is available. This leads to large safety marginsand to poor spectrum utilization, in general. Furthermore, the business success of future systems depends on thescalability of the secondary-access techniques. Resultsfrom the project indicate that in large-scale deployment,the aggregate interference from the secondary devices isthe key bottleneck. This aggregate interference is difficult for the individual secondary-spectrum user to assess. Inaddition, the vast majority of spectrum opportunities arestrongly dependent on the intended use. They are highly localized in time and space, and not obviously suitable for reliable service provisioning. The exception has been shown to be short-range, indoor communications, where the low transmitter power, walls, and other obstructions successfully provide these margins.