It is shown that both GaN and Ga0.8In0.2N quantum dots (QDs) can be grown by molecular beam epitaxy on silicon or sapphire substrates making use of the strain-induced two-dimensional (2D)-three dimensional (3D) growth mode occurring for mismatched materials (Stranski-Krastanov, SK, mode of growth). GaN and Ga0.8In0.8N QDs were embedded in an ALN and a GaN matrix, respectively. Despite the dislocation density (which can exceed 10(10) cm(-2) on silicon substrate), strong visible room temperature photoluminescence (PL) is observed owing to the QD related carrier localization and to the high QD density. Although GaN and AlN have band-gaps yielding to ultra-violet emission, the PL related to the GaN QDs is in the visible part of the electromagnetic spectrum. This is due to the presence of a large built-in electric field, which induces a strong quantum-confined Stark effect, and thereby an important red shift of the FL. It is demonstrated that the emission wavelength can be tuned in almost the whole visible spectrum range by simply varying the GaN or the GaInN QD size. The luminescence efficiency is found to be significantly larger in QD structures than in standard quantum well (QW) structures.