To realize an effective MIMO system, it is necessary to have a sufficient number of uncorrelated antennas at each end of the link. Mutual coupling or isolation from the adjacent antenna is a key factor to achieving high antenna performance in the MIMO antenna configuration. For low mutual coupling, the antennas must be far away from each other, but the space for the internal antenna is not enough to obtain a low correlation and mutual coupling in the mobile handsets. Moreover, it is very challenging to place multiple antennas within small mobile handsets while maintaining good isolation between the antenna elements because antennas can be strongly coupled with each other and even with the ground plane by sharing the surface currents distributed on the ground plane. In this book, various mutual coupling reduction techniques are introduced, and their mechanisms are analyzed. Several key performance parameters and the requirements for evaluating the MIMO antenna are also classified and analyzed. To reduce the mutual coupling, orthogonally polarized dipole elements, a band stop matching circuit, a decoupling network, and polarization diversity are utilized for MIMO antenna design.