Defect-engineered metal–organic frameworks (MOFs) and coordination network compounds (CNCs) are a promising area of research for tailoring material properties. Defects in MOFs and CNCs can be intentionally introduced or naturally occur, and their study is crucial for understanding and controlling physical and chemical properties such as band gaps, magnetic behavior, and electrical conductivity. Defect engineering involves the controlled manipulation of these defects to achieve desired material properties. This review discusses the classification, characterization, and potential applications of defects in MOFs and CNCs, as well as the importance of defect engineering in materials science. Defects in MOFs and CNCs can be categorized into point defects (e.g., vacancies), line defects (e.g., dislocations), planar defects (e.g., boundaries), and micro- and mesoscale volume defects (e.g., inclusions and voids). These defects can be internal or external and can significantly influence the material's properties. For example, linker vacancies can lead to the formation of coordinatively unsaturated metal sites (CUSs), which can have unique electronic, magnetic, and optical properties. Defect engineering can also enhance catalytic activity by creating active sites for reactions. Defects in MOFs and CNCs can be formed during synthesis or through postsynthetic treatments. For instance, solid-solution approaches involve mixing different organic linkers to create heterogeneous MOFs. Defect engineering can also be achieved through the introduction of metal node vacancies or by modifying the framework through acid/base treatments. These methods can lead to the formation of mesopores, which can improve mass transport and catalytic performance. The characterization of defects in MOFs and CNCs is essential for understanding their properties and applications. Techniques such as AFM, SEM, TEM, and XRD are used to image and analyze defects. Additionally, spectroscopic methods like FTIR and Raman spectroscopy can provide insights into the chemical and electronic properties of defective sites. Theoretical methods, including quantum mechanical/molecular mechanical (QM/MM) calculations and DFT simulations, are also used to model and predict the behavior of defects in MOFs and CNCs. Overall, defect engineering in MOFs and CNCs offers a powerful approach to tailor material properties for various applications, including catalysis, gas storage, and sensing. The study of defects in these materials is an active area of research with significant potential for future developments.Defect-engineered metal–organic frameworks (MOFs) and coordination network compounds (CNCs) are a promising area of research for tailoring material properties. Defects in MOFs and CNCs can be intentionally introduced or naturally occur, and their study is crucial for understanding and controlling physical and chemical properties such as band gaps, magnetic behavior, and electrical conductivity. Defect engineering involves the controlled manipulation of these defects to achieve desired material properties. This review discusses the classification, characterization, and potential applications of defects in MOFs and CNCs, as well as the importance of defect engineering in materials science. Defects in MOFs and CNCs can be categorized into point defects (e.g., vacancies), line defects (e.g., dislocations), planar defects (e.g., boundaries), and micro- and mesoscale volume defects (e.g., inclusions and voids). These defects can be internal or external and can significantly influence the material's properties. For example, linker vacancies can lead to the formation of coordinatively unsaturated metal sites (CUSs), which can have unique electronic, magnetic, and optical properties. Defect engineering can also enhance catalytic activity by creating active sites for reactions. Defects in MOFs and CNCs can be formed during synthesis or through postsynthetic treatments. For instance, solid-solution approaches involve mixing different organic linkers to create heterogeneous MOFs. Defect engineering can also be achieved through the introduction of metal node vacancies or by modifying the framework through acid/base treatments. These methods can lead to the formation of mesopores, which can improve mass transport and catalytic performance. The characterization of defects in MOFs and CNCs is essential for understanding their properties and applications. Techniques such as AFM, SEM, TEM, and XRD are used to image and analyze defects. Additionally, spectroscopic methods like FTIR and Raman spectroscopy can provide insights into the chemical and electronic properties of defective sites. Theoretical methods, including quantum mechanical/molecular mechanical (QM/MM) calculations and DFT simulations, are also used to model and predict the behavior of defects in MOFs and CNCs. Overall, defect engineering in MOFs and CNCs offers a powerful approach to tailor material properties for various applications, including catalysis, gas storage, and sensing. The study of defects in these materials is an active area of research with significant potential for future developments.