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We discuss the formation of dispersive shock waves of the spatial type occurring in defocusing media [1].When developing from a dark input waveform, the oscillations char acteristic of the shock waves behave as multiple pairs of dark solitons that emanate from a focus (shock) point.Stimulated by our experimental results obtained in a nonlocal (ther mal) medium, we have developed a full theory of the phenomenon.In the local limit, we establish a connection between the structure of the dispersive shock wave and the inte grability of the underlying wave equation, which allows us to develop a thermodynamical description of the soliton gas and to interpret the shock wave in terms of a phase-transition [2].Robustness of the phenomenon against nonlocality will be also discussed.