Cognitive Radio Networks (CRNs) are emerging as a solution to increase the spectrum utilization by using unused or less used spectrum in radio environments. The basic idea is to allow unlicensed users access to licensed spectrum, under the condition that the interference perceived by the licensed users is minimal. New communication and networking technologies need to be developed, to allow the use of the spectrum in a more efficient way and to increase the spectrum utilization. This means that a number of technical challenges must be solved for this technique to get acceptance. The most important issues are regarding Dynamic Spectrum Access (DSA), architectural issues (with focus on network reconfigurability), deployment of smaller cells and security.