A survey of the recent progress on the properties of transition-metal impurities in the most common III-V compounds GaAs, InP and GaP is presented. A summary of experimental techniques available for identifying deep centers and/or locating deep levels in the band gap is given. The author concentrates on 3d isolated impurities, giving the known energy levels of Fe-group substitutional impurities in the three compounds. Complexes involving these impurities are also considered. In addition, 4d, 5d and rare earth impurities are briefly discussed.