Recent results from studies of shallow donors, pseudodonors, and deep level defects in SiC are presented. The selection rules for transitions between the electronic levels of shallow donors in 4H-SiC in the dipole approximation are derived and the ionization energy for the N donor at hexagonal site is determined. Optical and electrical studies of the D-I center reveal the pseudodonor nature of this defect. Defects in high-purity semi-insulating (SI) SiC substrates including the carbon vacancy (V-C), silicon vacancy (V-Si), and (V-C-C-Si) pair are studied. The annealing behavior of these defects and their role in carrier compensation in SI 4H-SiC are discussed. (C) 2003 Elsevier B.V. All rights reserved.