Fourth-generation communications systems call for a high amount of computational power due to multiantenna and multimode features. The level of flexibility required is growing rapidly with the number of modes to be supported for a single protocol and the number of protocols to be supported by a single receiver. Such high level of flexibility becomes a key feature of new and legacy radio applications in many domains (military radio, broadcast systems, aeronautic communications, etc.), which call for adopting a software-defined radio (SDR) approach, or even for incorporating additional adaptive capabilities, such as suggested by cognitive radio (CR) research. In general, the design of flexible base-band platforms raises several critical problems, including the high level of required performance, the dissipated power, and the reconfiguration process itself. Several alternatives have been partially explored to implement flexible base-band building blocks and a lot of research is still required to bring efficiency into programmable platforms.