1. Introduction 2. Physics and theory of defects in two-dimensional materials: The role of reduced dimensionality 3. Defects in two-dimensional elemental materials beyond graphene 4. Defects in transition metal dichalcogenides 5. Efforts in realizing electronic grade graphene and h-BN 6. Efforts in realizing electronic grade transition metal dichalcogenide materials 7. Materials engineering: Defect passivation and healing 8. Nonequilibrium synthesis and processing approaches to tailor heterogeneity in 2D materials 9. Two-dimensional materials under ion irradiation: From defect production to structure and property engineering 10. Tailoring defects for 2D electrocatalysts 11. 2D defects and devices: Outlook and perspectives 12. Concluding remarks