Abstract: Hybrid wicking materials are used in high performance heat pipes where a bi-modal distribution of pore sizes offers advantages over a homogeneous monolithic porous structure. This wick is comprised of sintered metal powder formed on a foam, felt, screen or mesh metal substrate. A fine pore structure is formed by the metal powder while the substrate is comprised of large pores. The large pores are several times, preferably five times to several orders of magnitude larger in size than the small pores. The sintered powder metal and the metal substrate may be made of nickel, copper, molybdenum, niobium, aluminum, iron, cobalt, titanium and alloys based on these metals. This provides a wicking material with axial and radial variations in pore size for optimized performance under both horizontal and against gravity orientations.