CASTp 3.0 is a web server that provides online services for locating, delineating and measuring geometric and topological properties of protein structures, such as surface pockets, interior cavities and cross channels. It has been widely used since its inception in 2003. The latest version, CASTp 3.0, continues to provide reliable and comprehensive identifications and quantifications of protein topography. In addition, it now provides: (i) imprints of the negative volumes of pockets, cavities and channels, (ii) topographic features of biological assemblies in the Protein Data Bank, (iii) improved visualization of protein structures and pockets, and (iv) more intuitive structural and annotated information, including information of secondary structure, functional sites, variant sites and other annotations of protein residues. The CASTp 3.0 web server is freely accessible at http://sts.bioe.uic.edu/castp/. The CASTp server aims to provide comprehensive and detailed quantitative characterization of topographic features of proteins. Since its release 15 years ago, the CASTp server has ~45,000 visits and fulfills ~33,000 calculation requests annually. It has been proven to be a useful tool for a wide range of studies, including investigations of signaling receptors, discoveries of cancer therapeutics, understanding of mechanism of drug actions, studies of immune disorder diseases, analysis of protein–nanoparticle interactions, inference of protein functions and development of high-throughput computational tools. To provide additional useful information and to deliver improved user experience, we introduce here an updated server called CASTp 3.0. All important features of the previous versions of the server are retained, including detecting and characterizing cavities, pockets and channels of protein structures. In addition, we have substantially extended its functions by providing pre-computed topographic features of biological assemblies in the PDB database, as well as imprints of negative volumes of these topographic features. Furthermore, the user interface has been redesigned, making it more intuitive and informative. The new features of CASTp 3.0 include pre-computed results for biological assemblies, imprints of negative volumes of topographic features, and an improved user interface. The CASTp server takes protein structures in the PDB format and a probe radius as input for topographic computation. It identifies all surface pockets, interior cavities and cross channels in a protein structure and provides detailed delineation of all atoms participating in their formation. It also measures their exact volumes and areas, as well as sizes of the mouth openings if exist. These metrics are calculated analytically, using both the solvent accessible surface model (Richards' surface) and the molecular surface model (Connolly's surface). In addition, the CASTp server also provides imprints of topographic features. These results can be directly downloaded from CASTp server, which can be visualized using either the UCSF Chimera or ourCASTp 3.0 is a web server that provides online services for locating, delineating and measuring geometric and topological properties of protein structures, such as surface pockets, interior cavities and cross channels. It has been widely used since its inception in 2003. The latest version, CASTp 3.0, continues to provide reliable and comprehensive identifications and quantifications of protein topography. In addition, it now provides: (i) imprints of the negative volumes of pockets, cavities and channels, (ii) topographic features of biological assemblies in the Protein Data Bank, (iii) improved visualization of protein structures and pockets, and (iv) more intuitive structural and annotated information, including information of secondary structure, functional sites, variant sites and other annotations of protein residues. The CASTp 3.0 web server is freely accessible at http://sts.bioe.uic.edu/castp/. The CASTp server aims to provide comprehensive and detailed quantitative characterization of topographic features of proteins. Since its release 15 years ago, the CASTp server has ~45,000 visits and fulfills ~33,000 calculation requests annually. It has been proven to be a useful tool for a wide range of studies, including investigations of signaling receptors, discoveries of cancer therapeutics, understanding of mechanism of drug actions, studies of immune disorder diseases, analysis of protein–nanoparticle interactions, inference of protein functions and development of high-throughput computational tools. To provide additional useful information and to deliver improved user experience, we introduce here an updated server called CASTp 3.0. All important features of the previous versions of the server are retained, including detecting and characterizing cavities, pockets and channels of protein structures. In addition, we have substantially extended its functions by providing pre-computed topographic features of biological assemblies in the PDB database, as well as imprints of negative volumes of these topographic features. Furthermore, the user interface has been redesigned, making it more intuitive and informative. The new features of CASTp 3.0 include pre-computed results for biological assemblies, imprints of negative volumes of topographic features, and an improved user interface. The CASTp server takes protein structures in the PDB format and a probe radius as input for topographic computation. It identifies all surface pockets, interior cavities and cross channels in a protein structure and provides detailed delineation of all atoms participating in their formation. It also measures their exact volumes and areas, as well as sizes of the mouth openings if exist. These metrics are calculated analytically, using both the solvent accessible surface model (Richards' surface) and the molecular surface model (Connolly's surface). In addition, the CASTp server also provides imprints of topographic features. These results can be directly downloaded from CASTp server, which can be visualized using either the UCSF Chimera or our