W and WSi Ohmic contacts on both p- and n-type GaN have been annealed at temperatures from 300 to 1000 degrees C. There is minimal reaction (less than or equal to 100 Angstrom broadening of the metal/GaN interface) even at 1000 degrees C. Specific contact resistances in the 10(-5) Ohm cm(2) range are obtained for WSi, on Si-implanted GaN with a peak doping concentration of similar to 5 x 10(20) cm(-3), after annealing at 950 degrees C. On p-GaN, leaky Schottky diode behavior is observed for W, WSix and Ni/Au contacts at room temperature, but true Ohmic characteristics are obtained at 250-300 degrees C, where the specific contact resistances are, typically, in the 10(-2) Ohm cm(2) range. The best contacts for W and WSix are obtained after 700 degrees C annealing for periods of 30-120 s. The formation of beta-W2N interfacial phases appear to be important in determining the contact quality.