We have grown polycrystalline GaN on quartz, refractory metal (W, Mo, Ta and Nb) and Si substrates by using plasma-assisted molecular beam epitaxy. It has been found that polycrystalline GaN grown on quartz and refractory metal substrates shows a strong band-edge emission without yellow emission. GaN growth on Si with native oxide produces well c-orientated nanorods exhibiting a low field emission threshold and high emission current density. We will review the growth of polycrystalline GaN films and the evaluation of their optical properties and electron field emission characteristics. (C) 2005 Elsevier B.V. All rights reserved.