Protein-Protein Interaction Database (PPIDb) is a semi-automated, manually curated database which records information about interacting partner(s), oligomeric state and closest homologue for over 10,000 protein domains. These protein domains are selected from PASS2 database which is in direct correspondence with SCOP 1.75 release. Information is collected from PDB, PQS (now obsolete), PISA, PiQSi and PIMA server. Interaction information is recorded, wherever available, otherwise the protein domain is recorded to exist as a monomer. For all the recorded protein-protein interactions, PPIDb also contains information about the interface residues, total energies they contribute and the critical residues (hotspots) that provide stability/specificity to the binding sites.