This review discusses the defects and defect engineering in two-dimensional transition metal dichalcogenide (2D TMDC) materials. TMDCs are promising 2D materials with tunable electrical, optical, and magnetic properties. However, they contain intrinsic defects that limit their applications. Defects in TMDCs are classified into zero-dimensional (0D), one-dimensional (1D), and two-dimensional (2D) types. The review covers the fundamental structure of TMDCs, the types of defects, and existing defect engineering methods for both defect formation and reduction. It also explores the impact of defects on the electronic, optical, and magnetic properties of TMDCs. TMDCs have a layered structure with the general formula MX₂, where M is a transition metal and X is a chalcogen. The material has a sandwich structure with weak van der Waals forces between layers. The two main structural phases are 2H (trigonal prismatic) and 1T (octahedral symmetry). The bandgap of TMDCs changes with the number of layers, transitioning from indirect to direct as the number of layers decreases. The electronic, optical, and magnetic properties of TMDCs are influenced by defects, such as sulfur vacancies, anti-site defects, and grain boundaries. These defects can modulate the properties of TMDCs, and their control is crucial for applications in electronics and optoelectronics. Defects in TMDCs include point defects (e.g., sulfur vacancies, anti-site defects), line defects (e.g., grain boundaries), and planar defects (e.g., ripples, heterostructures). Point defects such as sulfur vacancies and anti-site defects can affect the electronic and optical properties of TMDCs. Line defects like grain boundaries can influence the electrical and optical properties of TMDCs. Planar defects such as ripples and heterostructures can alter the physical properties of TMDCs. Defect characterization tools for TMDCs include Raman spectroscopy, transmission electron microscopy (TEM), scanning tunneling microscopy (STM), and atomic force microscopy (AFM). Raman spectroscopy is used to determine the number of layers in TMDCs and to study the impact of defects on their properties. TEM is used to study the atomic structure and defects in TMDCs. The review also discusses the impact of defects on the properties of TMDCs and the methods used to engineer defects for desired applications.This review discusses the defects and defect engineering in two-dimensional transition metal dichalcogenide (2D TMDC) materials. TMDCs are promising 2D materials with tunable electrical, optical, and magnetic properties. However, they contain intrinsic defects that limit their applications. Defects in TMDCs are classified into zero-dimensional (0D), one-dimensional (1D), and two-dimensional (2D) types. The review covers the fundamental structure of TMDCs, the types of defects, and existing defect engineering methods for both defect formation and reduction. It also explores the impact of defects on the electronic, optical, and magnetic properties of TMDCs. TMDCs have a layered structure with the general formula MX₂, where M is a transition metal and X is a chalcogen. The material has a sandwich structure with weak van der Waals forces between layers. The two main structural phases are 2H (trigonal prismatic) and 1T (octahedral symmetry). The bandgap of TMDCs changes with the number of layers, transitioning from indirect to direct as the number of layers decreases. The electronic, optical, and magnetic properties of TMDCs are influenced by defects, such as sulfur vacancies, anti-site defects, and grain boundaries. These defects can modulate the properties of TMDCs, and their control is crucial for applications in electronics and optoelectronics. Defects in TMDCs include point defects (e.g., sulfur vacancies, anti-site defects), line defects (e.g., grain boundaries), and planar defects (e.g., ripples, heterostructures). Point defects such as sulfur vacancies and anti-site defects can affect the electronic and optical properties of TMDCs. Line defects like grain boundaries can influence the electrical and optical properties of TMDCs. Planar defects such as ripples and heterostructures can alter the physical properties of TMDCs. Defect characterization tools for TMDCs include Raman spectroscopy, transmission electron microscopy (TEM), scanning tunneling microscopy (STM), and atomic force microscopy (AFM). Raman spectroscopy is used to determine the number of layers in TMDCs and to study the impact of defects on their properties. TEM is used to study the atomic structure and defects in TMDCs. The review also discusses the impact of defects on the properties of TMDCs and the methods used to engineer defects for desired applications.