The Structural Classification of Proteins (SCOP) database provides a detailed and comprehensive description of the relationships of all known protein structures. It is organized into hierarchical levels: family, superfamily, fold, and class. Families are groups of proteins with significant sequence similarities or similar functions and structures. Superfamilies consist of proteins with low sequence identities but similar structures and functions. Folds describe the geometrical arrangement of secondary structures. Classes group folds based on their structural characteristics. SCOP includes all proteins from the Protein Data Bank (PDB) and many others with published descriptions but without coordinates. The classification is based on visual inspection and comparison of structures, ensuring accuracy. The unit of classification is usually the protein domain. Large proteins are classified by their individual domains. SCOP provides links to atomic coordinates, images, interactive viewers, sequence data, conformational changes, and literature references. It is freely accessible via the World Wide Web at http://scop.mrc-lmb.cam.ac.uk/scop/. Mirrors are available for easier access. SCOP allows users to search by sequence, keyword, or PDB identifier. It also offers browsing through the hierarchy of classes, folds, superfamilies, and families. SCOP serves as a powerful interface to the PDB, providing detailed information on structural and evolutionary relationships. It is useful for comparing individual structures with their evolutionary and structurally related counterparts. The highest levels of classification provide an overview of the diversity of protein structures, suitable for both researchers and students. SCOP is supported by various institutions and is available on multiple platforms.The Structural Classification of Proteins (SCOP) database provides a detailed and comprehensive description of the relationships of all known protein structures. It is organized into hierarchical levels: family, superfamily, fold, and class. Families are groups of proteins with significant sequence similarities or similar functions and structures. Superfamilies consist of proteins with low sequence identities but similar structures and functions. Folds describe the geometrical arrangement of secondary structures. Classes group folds based on their structural characteristics. SCOP includes all proteins from the Protein Data Bank (PDB) and many others with published descriptions but without coordinates. The classification is based on visual inspection and comparison of structures, ensuring accuracy. The unit of classification is usually the protein domain. Large proteins are classified by their individual domains. SCOP provides links to atomic coordinates, images, interactive viewers, sequence data, conformational changes, and literature references. It is freely accessible via the World Wide Web at http://scop.mrc-lmb.cam.ac.uk/scop/. Mirrors are available for easier access. SCOP allows users to search by sequence, keyword, or PDB identifier. It also offers browsing through the hierarchy of classes, folds, superfamilies, and families. SCOP serves as a powerful interface to the PDB, providing detailed information on structural and evolutionary relationships. It is useful for comparing individual structures with their evolutionary and structurally related counterparts. The highest levels of classification provide an overview of the diversity of protein structures, suitable for both researchers and students. SCOP is supported by various institutions and is available on multiple platforms.