Summary: This paper summarizes studies on focussing of melt flow into high porosity channels in the mantle, and the geochemical effects of such flow focussing. In particular, it is proposed that focussed porous flow plays an important role in formation of dunite channels and extraction of mid-ocean ridge basalt (MORB) from the upper mantle beneath spreading ridges. This process may be as important or more important than melt flow in open fractures. Recent results are reported from experimental studies on analog systems , and field studies of dunites formed by syn-tectonic focussing of porous flow along ductile shear zones . Then, new geochemical analyses of dunites and harzburgites from the mantle section of the Oman ophiolite are discussed. Dissolution of pyroxene (and some olivine) in ascending melt forms high-porosity channels for focussed melt extraction. Within such channels, high melt/rock ratios produce solid products near equilibrium with MORB. Diffuse flow of melt outside these channels at very low melt/rock ratios has a comparatively minor effect upon the composition of residual harzburgites.