MIMO radars have multiple transmit and receive channels. Such multichannel radars have been used in different configurations for many decades. On the one hand, the detection probability can be increased compared to a single channel if the antenna spacing is very large and many receivers are used. The fusion of many channels leads to a lower probability of failure than a single channel if the channels have different fading characteristics. On the other hand, especially with small antenna spacings, in the range of \(\lambda /2\) , many channels can be used for classical transmit or receive beamforming. This approach leads to so-called phased arrays. However, the MIMO systems presented in this chapter address a completely different mode of operation where the numerous channels are used to make the receive aperture appear larger than it actually is. In this context this is called a virtual aperture. Since the virtual aperture is larger than the real aperture, this approach leads to radars with an improved angular resolution. An important requirement for MIMO radars is that all transmit channels and all receive channels must be coherent, i.e. phase-locked to each other. This is the only way to create the virtual aperture, since the phase relationships between the antennas are evaluated. In addition, all transmit signals of each transmit channel must be orthogonal to each other to guarantee that the transmitted signals can be separated at the receiver. This chapter begins with an introduction of the virtual aperture and the creation of orthogonal signals. This is followed by a performance analysis of MIMO radars, along with a description of how to design and evaluate MIMO radars. Finally, the compensation of near field effects of large antenna arrays in the millimeter wave range is addressed.