The device technology for the wide-band-gap semiconductor group-III element nitrides (AIN, GaN, and InN) is much advanced over the growth of large-size, single-crystal line bulk material. In fact, lacking availability of thermal and lattice-matched substrates impedes the cost-effective fabrication of low-defect (low-loss) devices, which would bring a new quality in terms of device efficiency and lifetime. A promising route towards mass production of true bulk group-III element nitride crystal is the ammonothermal technique using supercritical ammonia under high-pressure conditions. Already demonstrated were the hydrothermal growth of a 3-in (0 0 0 1) ZnO crystal and, very recently, the first successful ammonothermal growth of a I -in (0 0 0 1) GaN crystal. Based on the history of low-temperature SiO2 and later ZnO growth by the hydrothermal technique, we report on recent achievements including solubility of GaN, and give an outlook for the growth of large-size GaN crystal by the ammonothermal route. (c) 2007 Elsevier B.V. All rights reserved.