Rayleigh fading converts an exponential dependency of the bit error probability on the average received bit energy-to-noise ratio into an inverse linear one, yielding a very large performance loss. Diversity is a remedy that provides the receiver with multiple independently faded replicas of the same information bearing signal. This chapter begins with a discussion of basic diversity methods and diversity combining techniques, including selective combining and maximal ratio combining. Multi-input multiple-output (MIMO) channels are then considered, and various types of MIMO systems discussed. The chapter concludes with the Alomouti transmit diversity scheme and spatial multiplexing.