This document provides an introduction to MIMO communications techniques and their applications in 3G and 4G wireless systems. It discusses the historical use of multiple antennas, advantages of MIMO such as increased data rates and spectral efficiency. It describes adaptive antenna techniques including beamforming and transmit/receive diversity. MIMO is defined as using multiple antennas at both the transmitter and receiver to increase spectral efficiency and data rates by transmitting independent data streams through parallel channels. The document provides examples of MIMO channel capacity and its advantages over SISO.