Hydrostatic extrusion is an efficient method of grain refinement to the nanometer scale in metallic materials. The paper shows that it can be used directly to obtain a mean grain size smaller than 100 nm with a significant fraction of high angle grain boundaries in aluminum alloys, titanium, and iron. It is also demonstrated that grain size reduction to this level in some other materials, e.g., nickel, requires a combination of hydrostatic extrusion (HE), as the final operation, after some other methods of severe plastic deformation (SPD). Grain refinement in metallic materials by HE has a significant effect on their properties with a significant increase in mechanical strength and improvement of wear and corrosion resistance while maintaining an acceptable level of plasticity.