STRIDE is a web server for secondary structure assignment from atomic resolution protein structures. It implements a knowledge-based algorithm that combines hydrogen bond energy and statistically derived backbone torsional angle information to produce assignments in maximal agreement with crystallographers' designations. The STRIDE web server provides access to this tool and allows visualization of the secondary structure, as well as contact and Ramachandran maps for any file uploaded by the user with atomic coordinates in the Protein Data Bank (PDB) format. A searchable database of STRIDE assignments for the latest PDB release is also provided. The STRIDE server is accessible from http://webclu.bio.wzw.tum.de/stride/. Secondary structure assignment is a major step in characterizing a newly determined protein structure. It serves as a basis for subsequent analyses, including visualization, structure comparison and classification, homology modelling, threading and sequence alignment. STRIDE was developed to accurately reproduce secondary structure designations created by human experts. It is a knowledge-based approach using a carefully verified set of secondary structural elements defined by crystallographers who have deposited structures in the PDB. The main difference between STRIDE and DSSP is that STRIDE considers both hydrogen bonding patterns and backbone geometry. The hydrogen bond energy is calculated using an empirical energy function which takes into account the distance between the donor and the acceptor and the deviations from linearity of the bond angles. A weighted product of hydrogen bond energy and torsion angle probabilities for α-helix and β-sheet is used to determine the start and stop positions of secondary structure elements based on empirically optimized recognition thresholds. The STRIDE web server, written in Python, provides all functions implemented in the STRIDE software and several additional visualization tools. It accepts as input atomic coordinates in standard PDB format, which can be either uploaded or pasted into a web form. The STRIDE home page offers options for basic secondary structure assignment, graphical representation of secondary structure assignment, contact map, Ramachandran plot, and a database of STRIDE assignments.STRIDE is a web server for secondary structure assignment from atomic resolution protein structures. It implements a knowledge-based algorithm that combines hydrogen bond energy and statistically derived backbone torsional angle information to produce assignments in maximal agreement with crystallographers' designations. The STRIDE web server provides access to this tool and allows visualization of the secondary structure, as well as contact and Ramachandran maps for any file uploaded by the user with atomic coordinates in the Protein Data Bank (PDB) format. A searchable database of STRIDE assignments for the latest PDB release is also provided. The STRIDE server is accessible from http://webclu.bio.wzw.tum.de/stride/. Secondary structure assignment is a major step in characterizing a newly determined protein structure. It serves as a basis for subsequent analyses, including visualization, structure comparison and classification, homology modelling, threading and sequence alignment. STRIDE was developed to accurately reproduce secondary structure designations created by human experts. It is a knowledge-based approach using a carefully verified set of secondary structural elements defined by crystallographers who have deposited structures in the PDB. The main difference between STRIDE and DSSP is that STRIDE considers both hydrogen bonding patterns and backbone geometry. The hydrogen bond energy is calculated using an empirical energy function which takes into account the distance between the donor and the acceptor and the deviations from linearity of the bond angles. A weighted product of hydrogen bond energy and torsion angle probabilities for α-helix and β-sheet is used to determine the start and stop positions of secondary structure elements based on empirically optimized recognition thresholds. The STRIDE web server, written in Python, provides all functions implemented in the STRIDE software and several additional visualization tools. It accepts as input atomic coordinates in standard PDB format, which can be either uploaded or pasted into a web form. The STRIDE home page offers options for basic secondary structure assignment, graphical representation of secondary structure assignment, contact map, Ramachandran plot, and a database of STRIDE assignments.