Preparing high‐quality 2D semiconductors represented by transition metal dichalcogenides (TMDCs) is of great importance for the next‐generation devices. However, currently available 2D TMDCs contain many atomic defects, which greatly affect their electronic and optical properties. This review starts with the formation of atomic defects and their effects on the properties of 2D TMDCs. Then, techniques for characterizing atomic defects are systematically summarized, including atomic‐resolution imaging and spectroscopy measurements. Further, recent progress on defect suppression during growth and defect repair after growth is reviewed. Finally, challenges and opportunities in this important field are discussed.