Surface and interface stresses in solids are crucial for understanding the thermodynamics and behavior of materials, especially when one or more dimensions are less than 10 nm. These stresses, defined as the reversible work per unit area needed to elastically stretch a surface or interface, significantly influence the equilibrium structure and behavior of solids. Surface stress is related to the surface free energy, which is the energy required to create a surface. Interface stress arises from the stretching of two phases relative to each other and can be associated with either stretching one phase or both equally. Theoretical calculations and experimental measurements show that surface and interface stresses affect thin films and surface reconstructions in metals. Surface stress is a second-rank tensor, while surface free energy is a scalar. The surface stress can be positive or negative, indicating tension or compression, respectively. Theoretical and experimental results support the idea that surface stress can lead to surface reconstructions, such as those observed in (111) oriented surfaces of Au and Pt. The Lagrangian coordinate system simplifies the analysis of surface stress by considering the change in area and surface free energy. The equilibrium of a small solid crystal is influenced by surface stress, leading to the Laplace pressure, which affects the chemical potential of the solid and surrounding fluid. The Laplace pressure for a solid is given by 2f/r, where f is the surface stress and r is the radius of the solid. Experimental measurements of surface stress involve measuring elastic strain and using Hooke's Law to determine the stress. Techniques such as electron diffraction and wafer bending have been used to measure surface stress in materials like Au, Ag, and Pt. The surface stress of reconstructed surfaces, such as the 7×7 Si(111) surface, has been measured and compared with theoretical calculations. Interface stress is associated with the stretching of two phases relative to each other or both equally. The interface stress can be calculated using the interfacial free energy and is influenced by the strain and elastic properties of the phases. Interface stress measurements in layered materials, such as crystalline polymers and organic crystals, have shown that interface stress can lead to compressive or tensile effects depending on the material structure. In thin films, surface and interface stresses can result in intrinsic stress, higher order elastic behavior, and affect the thermodynamics of epitaxy. Surface reconstructions in (111) oriented fcc metal surfaces are influenced by these stresses, leading to changes in surface structure and properties. The effects of surface and interface stresses on thin films are significant, as they determine the structural and mechanical behavior of these materials.Surface and interface stresses in solids are crucial for understanding the thermodynamics and behavior of materials, especially when one or more dimensions are less than 10 nm. These stresses, defined as the reversible work per unit area needed to elastically stretch a surface or interface, significantly influence the equilibrium structure and behavior of solids. Surface stress is related to the surface free energy, which is the energy required to create a surface. Interface stress arises from the stretching of two phases relative to each other and can be associated with either stretching one phase or both equally. Theoretical calculations and experimental measurements show that surface and interface stresses affect thin films and surface reconstructions in metals. Surface stress is a second-rank tensor, while surface free energy is a scalar. The surface stress can be positive or negative, indicating tension or compression, respectively. Theoretical and experimental results support the idea that surface stress can lead to surface reconstructions, such as those observed in (111) oriented surfaces of Au and Pt. The Lagrangian coordinate system simplifies the analysis of surface stress by considering the change in area and surface free energy. The equilibrium of a small solid crystal is influenced by surface stress, leading to the Laplace pressure, which affects the chemical potential of the solid and surrounding fluid. The Laplace pressure for a solid is given by 2f/r, where f is the surface stress and r is the radius of the solid. Experimental measurements of surface stress involve measuring elastic strain and using Hooke's Law to determine the stress. Techniques such as electron diffraction and wafer bending have been used to measure surface stress in materials like Au, Ag, and Pt. The surface stress of reconstructed surfaces, such as the 7×7 Si(111) surface, has been measured and compared with theoretical calculations. Interface stress is associated with the stretching of two phases relative to each other or both equally. The interface stress can be calculated using the interfacial free energy and is influenced by the strain and elastic properties of the phases. Interface stress measurements in layered materials, such as crystalline polymers and organic crystals, have shown that interface stress can lead to compressive or tensile effects depending on the material structure. In thin films, surface and interface stresses can result in intrinsic stress, higher order elastic behavior, and affect the thermodynamics of epitaxy. Surface reconstructions in (111) oriented fcc metal surfaces are influenced by these stresses, leading to changes in surface structure and properties. The effects of surface and interface stresses on thin films are significant, as they determine the structural and mechanical behavior of these materials.