In this chapter, we will discuss how the performance will be in RIS-aided MIMO communications. In Sect. 2.1, we consider the physical constraint of RIS, where the number of the phase shift for each element is limited. In Sect. 2.2, we consider how many reflective elements of the RIS can offer an acceptable data rate. These two sections focus on the point-to-point communications, followed by multi-user communications. In Sect. 2.3, we optimize the placement and orientation of the RIS to extend the coverage. In Sect. 2.4, a hybrid beamforming scheme is introduced in the RIS-aided MIMO communications. Finally, in Sect. 2.5, the hybrid type RIS is utilize to achieve a full-dimensional coverage extension.