We propose Joint Spatial Division and Multiplexing (JSDM), an approach to multiuser MIMO downlink that exploits the structure of the channel vectors correlation in order to allow for a large number of base station antennas while requiring a considerably reduced channel state information (CSI) feedback from the users. Thanks to the reduced CSI requirement, JSDM allows the use of a large number of base station antennas also in FDD systems. This work presents the correlated channel model, the main idea of JSDM precoding, and the design of JSDM precoding, formed by the concatenation of a pre-beamforming matrix based only on the structure of the channel covariance, with suitable multiuser precoding based on the CSI of the reduced dimensional transformed channels. We show that in some cases JSDM incurs no loss of optimality with respect to the MIMO broadcast channel with full CSI.