Flexible crystals have attracted significant attention due to their remarkable pliability, plasticity, and adaptability, making them valuable in various research and application fields. The development of flexible crystals involves challenges in rational design, preparation, and performance optimization. Understanding the fundamental origins of crystal flexibility is crucial for establishing evaluation criteria and design principles. This Perspective reviews the development of flexible crystals over the past two decades, summarizing elastic standards and possible plastic bending mechanisms tailored to diverse flexible crystals. It analyzes the theoretical basis and applicability of these crystals, discussing the compatibility between crystal elasticity and plasticity, and highlighting their potential applications in biomedicine, flexible electronics, and optics. The article presents state-of-the-art experimental avenues and analysis methods for crystals, which are vital for future exploration of crystal flexibility mechanisms. Flexible crystals can undergo elastic or plastic deformation. Elastic deformation refers to the ability of crystals to deform under mechanical force and regain their original shape without permanent deformation. Plastic deformation, on the other hand, involves irreversible deformation that cannot recover the original shape after the force is removed. Plastic deformation can occur through molecular layer sliding, where molecular layers slip over each other due to reorganization of weak intermolecular interactions. Elastic deformation is often attributed to interlocking stacking and multiple weak interactions. Some crystals exhibit both elastic and plastic deformation, demonstrating the compatibility of these properties. The mechanisms of plastic and elastic deformation are influenced by molecular stacking, intermolecular interactions, and the presence or absence of slip planes. The isotropic interlocking stacking model, fibril lamella morphology model, and reversible molecular rotation model are key mechanisms explaining elastic deformation. These models highlight the importance of molecular stacking patterns, weak intermolecular interactions, and the absence of slip planes in achieving elasticity. The study of these mechanisms provides insights into the design and development of flexible crystals with advanced mechanical properties.Flexible crystals have attracted significant attention due to their remarkable pliability, plasticity, and adaptability, making them valuable in various research and application fields. The development of flexible crystals involves challenges in rational design, preparation, and performance optimization. Understanding the fundamental origins of crystal flexibility is crucial for establishing evaluation criteria and design principles. This Perspective reviews the development of flexible crystals over the past two decades, summarizing elastic standards and possible plastic bending mechanisms tailored to diverse flexible crystals. It analyzes the theoretical basis and applicability of these crystals, discussing the compatibility between crystal elasticity and plasticity, and highlighting their potential applications in biomedicine, flexible electronics, and optics. The article presents state-of-the-art experimental avenues and analysis methods for crystals, which are vital for future exploration of crystal flexibility mechanisms. Flexible crystals can undergo elastic or plastic deformation. Elastic deformation refers to the ability of crystals to deform under mechanical force and regain their original shape without permanent deformation. Plastic deformation, on the other hand, involves irreversible deformation that cannot recover the original shape after the force is removed. Plastic deformation can occur through molecular layer sliding, where molecular layers slip over each other due to reorganization of weak intermolecular interactions. Elastic deformation is often attributed to interlocking stacking and multiple weak interactions. Some crystals exhibit both elastic and plastic deformation, demonstrating the compatibility of these properties. The mechanisms of plastic and elastic deformation are influenced by molecular stacking, intermolecular interactions, and the presence or absence of slip planes. The isotropic interlocking stacking model, fibril lamella morphology model, and reversible molecular rotation model are key mechanisms explaining elastic deformation. These models highlight the importance of molecular stacking patterns, weak intermolecular interactions, and the absence of slip planes in achieving elasticity. The study of these mechanisms provides insights into the design and development of flexible crystals with advanced mechanical properties.