Various sclerostomy procedures, performed ab interno and ab externo have found increasing interest for some time. With few exceptions, invasive procedures are described, in which laser energy is transmitted by means of specially designed fibers and probes ab interno and ab externo to the perforation site. Laser energy sources, ranging from the UV through the visible to the IR range have been described. Thus far, no detailed models quantifying the ablation mechanism of such sclerostomy procedures have been published. All such procedures have in common that the wavelength of the laser should be matched to water absorption bands. Now more sophisticated models describing the action mechanism of various lasers begin to emerge. A system described by us takes advantage of the high efficiency of rapid steam explosions occurring in a closed space, i.e. in the region delimited by the probe end and the fistula bottom. The high efficiency of such mechanisms has been verified in model experiments on porcine eyes in vitro. Electron microscopical results also show that the amount of thermal damage is negligible and that there is very little wavelength-dependence. This permits the adoption of "common" laser energy sources such as diode and Nd:YAG lasers. The clinical survival probability of fistulas as related to the amount of collateral damage is controversial.