Cognitive radio networks enable unlicensed users to communicate using the licensed spectrum without causing interference to legitimate users. To establish a communication link, two or more cognitive users must simultaneously visit a common available channel and exchange the handshake information, such a process is referred to as rendezvous. The simplest solution to the rendezvous problem is to use a dedicated common control channel. However, it can suffer severe congestion or even be occupied by incumbent users, limiting the functioning of the cognitive network. Channel hopping (CH) techniques overcome these drawbacks and allow cognitive users to rendezvous on any common available channel. In this paper a classification framework is proposed based on relevant features for the design of CH rendezvous sequences. Using the proposed framework, an unprecedented number of CH sequences are reviewed and classified. Theoretical performance metrics of existing CH sequences are summarized. Finally, open issues and research directions for CH sequences design are discussed.