Current generation sonar systems employ high temporal and spatial resolution to achieve acceptable operational performance. This, together with the physical mechanisms of acoustic propagation in the ocean, places a number of constraints on the implementation of beam-forming systems for sonar applications that are in many ways unique; in particular:(a) Low sonar operating frequencies and the use of wide processing and analysis bandwidths necessitate the use of broadband beam-forming techniques, often using true time-delay rather than phase-delay realisation.(b) Large arrays of sensors are required for high spatial resolution and the beam-forming system must be capable of forming and processing from these large numbers of independently steered beams in parallel to achieve useful angular cover, as well as good angular resolution.(c) The sensor arrays themselves are often mounted on mobile platforms, for example on surface ships, and the effects of platform motion on array data must be considered if the intrinsic system resolution is to be maintained.(d) The array geometries employed are often constrained by platform requirements rather than by those of the sonar and consequently sonar beam-forming systems must be able to process data from various diverse array geometries, for example from conformal and cylindrical, as well as planar sensor arrays. This paper outlines the impact of such factors on sonar beam-forming implementations and describes some of the techniques that have been developed, together with their practical realisation using state-of-the-art digital l.s.i. technology.