The aqueous leaves the anterior segment via two different pathways, the classical and the uveoscleral routes. The main resistance to aqueous egress lies at the level of the juxtacanalicular trabeculum and the inner wall of Schlemm's canal. Impaired function of either one of these structures leads to a decrease in the outflow facility which eventually results in a significant increase in intraocular pressure. Success of non-penetrating surgery lies in the physical removal of these resisting elements without compromising of the structural integrity of the eye, thus preventing a sudden drop in the intraocular pressure. The selective filtration through the trabeculodescemetic membrane, the evacuation of the aqueous in the intrascleral space, the subconjunctival bleb and the subchoroidal space, and the final drainage into collector channels and drain veins are the key elements for a successful deep sclerectomy.