This lecture covers the most common defects found in 2D materials, including point defects like vacancies, substitutions, and dopants, as well as line defects such as grain boundaries and lateral heterostructures. The impact of defects on the atomic and electronic structure of materials is discussed, along with strategies for defect mitigation through dielectric environments and chemical treatments. The lecture also explores the positive applications of defects, such as reducing contact resistance, creating single-photon sources, and inducing magnetism.