In this monograph, investigations of the electronic structures of the transition-metal oxides MnO, CoO, and NiO with spin-polarized electron energy-loss spectroscopy are presented and compared with other experimental and theoretical results. After a review of the present knowledge of the electronic structure of the monoxides, the spectroscopic method applied and its special advantages are described. The knowledge and use of the different spin, angle, and primary-energy dependences of the various relevant inelastic electron-scattering mechanisms provide new insights into the excitation processes of the optically forbidden transitions between the crystal-field-split 3d states of the bulk and of the surface.