One of the technical goals in underwater acoustics is to design coatings to reduce the acoustic echo from submerged structures to avoid detection versus active sonar systems. At sufficiently high frequencies, this can be done using passive materials. At low frequencies, however, the task is more difficult, and active systems are under consideration. The purpose of this paper is to summarize the different principles of acoustic active control that can be considered and to point out advantages and difficulties, with a few examples. They include: (1) piezoelectric (pseudo-passive) coatings; (2) control of the impedance of a piezoelectric transducer; (3) distributed array of local sensor/actuator systems; (4) combined sensor and actuator arrays.