Extensive studies on laser lithotripsy of urinary and gall calculi using a microsecond pulsed dye laser have yielded information on the mechanism of plasma initiation, expansion and shock wave fragmentation relevant to the determination of optimum fragmentation conditions. The key to plasma formation lies in the initial absorption of laser energy by the calculus surface to produce a sufficient density of localized vapour to absorb strongly laser energy. The subsequent ionization of the vapour by the further absorption of laser energy leads to plasma formation. The absorbance of several calculi pigmentations have been measured giving an indication of the individual susceptibility to laser induced breakdown. The plasma threshold is measured to be fluence dependence, which is caused by the vaporization and the further heating of the vapour. The transient expansion of the plasma imparts an impulse on stone and induces a sharp shock wave with peak pressure as high as 10 kilobar leading to the fragmentation of calculi.