The seven papers included in this special section focus on Cognitive radio ad hoc networks (CRAHNS). CRAHNs are equipped with the intrinsic capabilities of cognition and self-organization, providing a flexible spectrum- aware communication paradigm in wireless networks. Since available spectrum mostly manifests as several discontinuous frequency ranges, CRAHNs should be able to operate on multiple frequency channels. Multi-channel CRAHNs can alleviate interference and improve the spectrum utilization with greater fl exibility for channel access. The promising applications of multi-channel CRAHNs include large-scale machine-to-machine (M2M) communication, the Internet of Things (IoTs), high-density Wi-Fi, and unmanned aerial vehicle (UAV) swarms. Multi-channel CRAHNs impose unique challenges due to the distributed dynamic network topology and the temporal or spatial variations in spectrum availability. To overcome these challenges, fundamental problems in multi-channel CRAHNs have to be carefully resolved with respect to spectrum sensing, convention resolution, network topology maintenance, and application support.