The chapter discusses the concept of dislocations in materials, emphasizing their importance in understanding material behavior and plastic deformation. Dislocations are one-dimensional defects that play a crucial role in the weakening of crystals and the process of plastic deformation. The text covers various types of dislocations, including edge dislocations, screw dislocations, and mixed dislocations, and explains how they interact with other defects and external constraints. Key points include: - The role of dislocations in weakening crystals and the formation of steps. - The determination of Burgers vectors and their significance. - The motion of dislocations, including climb and cross-slip. - The stress fields associated with dislocations and their interaction with other defects. - The energy associated with dislocations and the dissociation of dislocations. - The interaction between dislocations, such as attractive and repulsive forces. - The presence of dislocations in different crystal structures, including CCP crystals. - The concept of image forces near free surfaces and their impact on dislocation movement. The chapter also includes examples and solved problems to illustrate the theoretical concepts, such as determining Burgers vectors, evaluating dislocation reactions, and calculating image forces.The chapter discusses the concept of dislocations in materials, emphasizing their importance in understanding material behavior and plastic deformation. Dislocations are one-dimensional defects that play a crucial role in the weakening of crystals and the process of plastic deformation. The text covers various types of dislocations, including edge dislocations, screw dislocations, and mixed dislocations, and explains how they interact with other defects and external constraints. Key points include: - The role of dislocations in weakening crystals and the formation of steps. - The determination of Burgers vectors and their significance. - The motion of dislocations, including climb and cross-slip. - The stress fields associated with dislocations and their interaction with other defects. - The energy associated with dislocations and the dissociation of dislocations. - The interaction between dislocations, such as attractive and repulsive forces. - The presence of dislocations in different crystal structures, including CCP crystals. - The concept of image forces near free surfaces and their impact on dislocation movement. The chapter also includes examples and solved problems to illustrate the theoretical concepts, such as determining Burgers vectors, evaluating dislocation reactions, and calculating image forces.