This chapter is focused on defining and optimizing an energy-efficiency metric for MIMO systems. This metric, which expresses in bit per Joule, allows one to measure how much information is effectively transferred to the transmitter per unit cost of energy consumed at the transmitter. For a MIMO point-to-point communication (single user MIMO channels) this metric can be useful to determine what power level, precoding scheme, training length, or number of antennas have to be used for obtaining the maximum information that is effectively transferred per unit energy spent. Then, we move from a physical layer-type approach to a cross-layer design of energy-efficient power control by including the effects a queue with finite size at the transmitter. As a last step we study a distributed multiple user scenario (MIMO multiple access channels) where each user selfishly maximizes its energy-efficiency by choosing its best individual power allocation policy. Here, we present the most relevant results in this field in a concise and comprehensible manner.