The article by Robert C. Cammarata discusses the role of surface and interface stresses in the thermodynamics of solids, particularly focusing on thin films. Surface stress is defined as the reversible work per unit area needed to elastically stretch a pre-existing surface, while interface stress is associated with the reversible work per unit area needed to strain one phase relative to another at an interface. The author reviews theoretical calculations and experimental measurements to illustrate the physical meaning and significance of these quantities. Key points include: 1. **Surface Stress**: Surface stress is a second-rank tensor that can be positive (tensile) or negative (compressive). It affects the equilibrium structure and behavior of solids, especially in thin films. 2. **Interface Stress**: Interface stress is associated with solid-solid interfaces and can be categorized into two types: one involving the strain of one phase relative to another, and the other involving the equal stretching of both phases. 3. **Thermal and Mechanical Effects**: Surface and interface stresses can induce significant intrinsic stress, higher-order elastic behavior, and affect the thermodynamics of epitaxy in thin films. 4. **Experimental Methods**: Experimental measurements of surface and interface stresses are discussed, including techniques such as electron diffraction and wafer bending. 5. **Thin Films**: The effects of surface and interface stresses on thin films are highlighted, including strain-induced elastic moduli variations and higher-order elastic effects. The article provides a comprehensive overview of the theoretical and experimental aspects of surface and interface stresses, emphasizing their importance in understanding the behavior of materials at the nanoscale.The article by Robert C. Cammarata discusses the role of surface and interface stresses in the thermodynamics of solids, particularly focusing on thin films. Surface stress is defined as the reversible work per unit area needed to elastically stretch a pre-existing surface, while interface stress is associated with the reversible work per unit area needed to strain one phase relative to another at an interface. The author reviews theoretical calculations and experimental measurements to illustrate the physical meaning and significance of these quantities. Key points include: 1. **Surface Stress**: Surface stress is a second-rank tensor that can be positive (tensile) or negative (compressive). It affects the equilibrium structure and behavior of solids, especially in thin films. 2. **Interface Stress**: Interface stress is associated with solid-solid interfaces and can be categorized into two types: one involving the strain of one phase relative to another, and the other involving the equal stretching of both phases. 3. **Thermal and Mechanical Effects**: Surface and interface stresses can induce significant intrinsic stress, higher-order elastic behavior, and affect the thermodynamics of epitaxy in thin films. 4. **Experimental Methods**: Experimental measurements of surface and interface stresses are discussed, including techniques such as electron diffraction and wafer bending. 5. **Thin Films**: The effects of surface and interface stresses on thin films are highlighted, including strain-induced elastic moduli variations and higher-order elastic effects. The article provides a comprehensive overview of the theoretical and experimental aspects of surface and interface stresses, emphasizing their importance in understanding the behavior of materials at the nanoscale.