Group-III nitride quantum dots (QDs) are grown by molecular beam epitaxy. GaN/AIN QDs are used as a prototypical system for studying the interplay of the carrier localization and the built-in polarization field. The latter effect pushes the QD photoluminescence (PL) in the red range. InGaN/GaN QDs are fabricated taking advantage of the Stranski-Krastanow growth mode transition. In0.15Ga0.85N/GaN QDs exhibit a superior radiative efficiency compared to low In content (x less than or equal to0.1) InxGa1-xN/GaN quantum wells (QWs). However, the optical properties of InGaN/GaN QWs with an In composition of 20% are very close to those of In0.15Ga0.85N/GaN QDs. These similarities could be related to strong In composition fluctuations forming deep potential wells for x greater than or equal to0.15. Blue to red PL at room temperature is then observed on In0.2Ga0.8N/GaN QWs despite the huge density of dislocations. 0.4 to 0.6 mum electroluminescence is obtained from InGaN/GaN based light emitting diodes.