We have used EPR and ODMR to study state-of-the-art bulk ZnO single crystals. Most of the samples are n-type, however, under certain conditions (e-irradiated or annealed), we observe a center due to residual nitrogen (g‖ = 1.9953, g┴ = 1.9633 and A(iso) = 1.225 mT, A(aniso) = 0.864 mT). The N center is a nitrogen p-orbital along the c-axis, very similar to P or As in ZnSe, which have a Jahn-Teller distorted, deep state, seen in EPR. The intensity of the N center is strongly anisotropic, as would be expected for shallow heavy-hole acceptors with m(J) = +/- 3/2 in which the magnetic dipole transition is forbidden for B‖c and becomes allowed away from that orientation due to mixing with m(J) = ± 1/2 states. The EPR spectra for the n-type samples are dominated by delocalized donors (g‖ = 1.9570 and g┴ = 1.9551). In one sample, the donor appears to be localized. These crystals also exhibit a broad PL band at 2.5 eV. ODMR on the 2.5 eV ''green'' PL band reveals an S= 1 triplet center (not seen in EPR) with spin-Hamiltonian parameters of D = 0.763 GHz, g‖ = 1.9710, and g┴= 2.0224. ODMR of shallow donors is also observed as a luminescence-decreasing feature on the 2.5 eV green emission band. Published by Elsevier Science B.V.