ZnO has ideal qualities for bright, efficient UV light emitting diodes and laser diodes, based on p-n junctions. However, while high quality n-type ZnO has been available for many years, the development of good p-type material is a much more recent phenomenon. The most successful acceptor dopants have been the group V elements, N, P and As; N substitutes on the O site, but the exact structures of the P and As acceptors have not yet been established. Resistivities as low as 0.4 Omega cm have been measured, and some UV heterojunction and homojunction LEDs have been fabricated. Optical fingerprints of p-type ZnO often include a photoluminescence line at 3.31 eV, and strong donor-bound exciton lines at 3.357 and 3.367 eV, both of which are well known from previous studies of n-type ZnO.