In sonar, we are concerned with the detection of targets against a background of unwanted signals. These unwanted signals may be noise in several forms, or reverberation. The task of the receiving system designer is to ensure an optimal system response to the wanted signal and minimal response to the unwanted signal. The receive array can play an important part in this process. Sonar suffers from noise problems due to its use of sound waves whose influence on the receiving sensor may be indistinguishable from that due to other mechanical sources. Discrimination against vibration for example may be best achieved in the sound sensor itself rather than in the signal processing. This chapter is concerned with the very early stages in the sonar detection process.