The database of three-dimensional interacting domains (3did) is a biological database containing a catalogue of protein-protein interactions for which a high-resolution 3D structure is known. 3did collects and classifies all structural models of domain-domain interactions in the Protein Data Bank, providing molecular details for such interactions. The current version also includes a pipeline for the discovery and annotation of novel domain-motif interactions. For every interaction 3did identifies and groups different binding modes by clustering similar interfaces into “interaction topologies”. By maintaining a constantly updated collection of domain-based structural interaction templates, 3did is a reference source of information for the structural characterization of protein interaction networks. 3did is updated every six months and is available for bulk download and for browsing at http://3did.irbbarcelona.org.