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Dynamic spectrum access brings an emerging paradigm shift in improving spectrum utilization significantly from the conventional exclusive allocation and usage of spectrum resource. It allows secondary unlicensed users to dynamically share the spectrum with primary licensed users without affecting the normal spectrum utilization in the primary systems. Cognitive radio (CR) has become an important technology to promote dynamic spectrum access by opportunistically sensing and exploiting the spectrum "holes." Meanwhile cooperative relay technology brings another paradigm shift in wireless communication and signal processing by introducing cooperative diversity through distributed transmission and processing with user and node collaboration. In the context of cognitive radio systems, cooperative communication is inspiring new design concepts and bringing new dimensions to dramatically improve the spectrum utilization and efficiency in wireless networks.