  Information-theoretic limits of {\it cognitive radio} networks have been
under exploration for more than a decade. Although such limits are unknown for
many networks, including the simplest case with two pairs of
transmitter-receiver, there are several cases for which the capacity limits are
obtained either exactly or up to a constant gap. The goal of this chapter is to
provide insights into the nature of transmission techniques associated with
optimal communication when cognitive radio technology is used. %, with an
emphasis on 5G networks. Outlining the state of the art in the
information-theoretic analysis of different cognitive systems, we highlight the
salient features/points of the capacity-achieving or capacity-approaching
strategies that should be considered in wireless network design paradigms based
on this technology. In particular, we emphasize the interaction of cognitive
radio with emerging technologies for 5G networks.
