The multiple-input multiple-output (MIMO) model represents several communication systems encompassing: CDMA systems, multiple array systems, and multicarrier systems such as OFDM, DMT, filter-bank-based multicarrier systems, and many combinations of optimized MIMO systems. This chapter discusses many aspects of MIMO systems, starting with their general description, including several distinct configurations of communication systems. Then, a review of the basic concepts of statistical signal processing is introduced to provide tools to design some transceivers. These concepts are applied to design precoders and equalizers in the transceivers.