last decades witnessed several research activities in the area of acoustic undersea warfare targeting the development of advanced systems to accurately detect and localize underwater moving targets. One of the main categories of these systems is the passive sonar (sound navigation and ranging) that searches for the location of the ships and submarines by listening to the radiated noise produced by their propellers, machinery, and flow dynamics. The performance of the passive sonar (involving estimation of the target bearing) highly depends on the particular array signal processing algorithms used in practice. Presently, the main challenge is to accurately estimate the target bearing in low signal to noise ratio for the underwater environment. This book presents advanced high spatial resolution techniques for both uniform and nonuniform hydrophone arrays. These techniques are now utilized in real applications due to its outstanding performance.