This chapter systematically classifies and analyzes defects in AlN single crystals, emphasizing their structural, thermodynamic, and kinetic origins. Point defects, including intrinsic vacancies, interstitials, and extrinsic dopants (e.g., C, Mg), are analyzed with respect to their formation energies and electronic states via first-principles calculations. Dislocations in AlN, classified by Burgers vectors and slip systems, highlight those formed during heteroepitaxial growth. The formation, motion, and interaction of threading dislocations are influenced by lattice mismatch relaxation, surface-mediated climb mechanisms, substrate miscut angles, and image forces from free surfaces. Stacking faults and grain boundaries are explored in the context of growth on non-polar planes and twin structures. Understanding the pivotal role of these defects in shaping AlN’s properties provides insights into enhancing device performance through targeted defect engineering.