In recent years, with an ever increasing number of wireless services to be allocated within a limited frequency resource, much research has focused on the development of spectrally efficient transmission technologies at the physical layer. The most promising among these technologies is that of MIMO systems, which prescribe radio links with multiple antennas at both ends. A critical factor in determining the performance of a MIMO system is the availability of some information about the propagation channel (Channel State Information, CSI) at the receiver and, possibly, at the transmitter. This thesis attempts to fill a gap in the present body of work on this subject by addressing CSI acquisition and the design of linear precoding and equalization with imperfect CSI. The main contribution is the study of these issues within a realistic frequency-selective channel model, suitable for different propagation environment and antenna geometries.