The high nitrogen pressure solution (HNPS) method allows the growth of GaN single crystals of very high structural quality, with a dislocation density as low as 10-100 cm(-2). The crystals are hexagonal platelets with a (0001) surface area of 1 cm(2) and a thickness of 0.1 mm. Both highly conductive and non-conductive GaN crystals can be grown by the HNPS method. The conductive crystals are grown without intentional doping, whereas the non-conductive are intentionally magnesium-doped during crystallization. It will be shown that atomically flat epi-ready N-polar surfaces can be obtained by mechanical and mechano-chemical polishing with the use of alkaline water solutions. The opposite Ga-polar surface is chemically inert to room temperature wet etching, and has to be treated by reactive ion etching to remove subsurface damage introduced by mechanical polishing. The crystals were used for the epitaxy of GaN by molecular beam epitaxy (MBE) with both N-2-plasma and NH3 nitrogen sources. In most MBE studies, the N-polar surfaces were used. Both methods allow 2D-growth of near dislocation-free GaN homoepitaxial layers and multi-quantum well (MQW) structures.