The present project proposal aims at studying, develop and test in the field an underwater acoustic barrier UAB). The objective of an UAB is to detect underwater intrusions in a given area of the ocean, like for instance, a bay, a port or a river. One of the most promising concepts in this area today is to form a virtual plane between two vertical acoustic transmitting/receiving line arrays of transducers/receivers, the sea surface and the bottom, using the acoustic time-reversal mirror (TRM) principle. The two vertical line arrays (VLA) are connected either by cable or by a wireless lan. Forming a focused TRM on each of the receivers of the VLA will allow, in principle, the detection of any acoustic reflecting object perturbing the acoustic field when crossing the barrier. There are two type of problems to be treated in this project: theoretical problems dealing with the optimal frequency and number and distribution of sensors of the VLA’s allowing a adequate resolution of the targets and technological problems dealing with the assembling of the system formed by the two synchronized VLA’s being able to work on real time and in shallow waters (20/30 m).