In this chapter, array and beamformer are used interchangeably. When two or more signals are added, they may be added constructively if they are in phase or destructively if they are out of phase. The term array in this chapter refers to a transducer array. A transducer array is a set of transducers where each transducer element is connected to a phase shifter and a gain scaler in such a way that when the signal impinging on (or radiating from) the array is added together, the signals from (or radiating to) a particular direction is added constructively but those in other directions are added destructively to achieve spatial selectivity.