Cognitive Radio Ad-Hoc Networks (CRAHNs) are a class of cognitive radio networks. In recent years, they have gained popularity, and routing protocols have been proposed. Above all, the protocols based on on-demand routing are considered favorable in the literature. It is mainly because the accomplishments of Ad-hoc On-demand Distance Vector (AODV) routing and Dynamic Source Routing (DSR) in mobile ad hoc networks have lead to a number of adaptations of both protocols to suit CRAHNs. In this paper, the authors review the on-demand routing protocols applicable for CRAHNs, which are based on AODV, DSR, and hybrid protocols.