In the last decade there has been considerable research activity in doping wide band gap semiconductors with rare earth ions (RE). Eu, Er, and Tm doping leads to the emission of visible light in narrow bands in the red, green, and blue, respectively. This allows for the realization of high brightness light emitters with outstanding durability and long lifetime. Nevertheless, the knowledge of the mechanisms underlying the RE active ion luminescence is required for the realization of efficient devices. The rare earth incorporation, i.e. the RE lattice location, the position of the energy levels of the RE with respect to the host valence and conduction bands, as well as a detailed investigation of the involved energetic relationships have to be determined and understood. Here we review the results of state of the art first-principles investigations on Eu, Er and Tm doping in GaN. The results of the simulations, including lattice location, defect symmetry, and electronic configuration of the RE are presented and compared with experimental results.