The atomic-scale origin of the unusually high performance and long-term stability of wurtzite p-GaN oriented nanowire arraysis revealed. Nitrogen termination of both the polar (000 (1) over bar) top face and the non polar (10 (1) over bar0) side faces of the nanowires is essential for long-term stability and high efficiency. Such a distinct atomic configuration ensures not only stability against (photo) oxidation in air and in water/electrolyte but, as importantly, also provides the necessary overall reverse crystal polarization needed for efficient hole extraction in p-GaN.