Cognitive radio constitutes a very promising solution to improve spectrum usage of communication systems. However, being equipped with MIMO antennas, the design of CR devices poses tremendous challenges. In this paper, antennas requirements as well as the limitations imposed on the system performance are reviewed. Well decoupled and reconfigurable antennas guarantee the flexibility needed during spectrum scanning and data transmission. Nevertheless to achieve the desired performance, many challenges have to be overcome. The different aspects are illustrated by an example of a 4-port MIMO antenna in the frequency range [470, 790] MHz, or the so-called TV white spaces.