Improvements in nucleotide sequencing technology have resulted in an ever increasing number of nucleotide and protein sequences being deposited in databases. Unfortunately, the ability to manually classify and annotate these sequences cannot keep pace with their rapid generation, resulting in an increased bias toward unannotated sequence. Automatic annotation tools can help redress the balance. There are a number of different groups working to produce protein signatures that describe protein families, functional domains or conserved sites within related groups of proteins. Protein signature databases include CATH-Gene3D, HAMAP, PANTHER, Pfam, PIRSF, PRINTS, ProDom, PROSITE, SMART, SUPERFAMILY, and TIGRFAMs. Their approaches range from characterising small conserved motifs that can identify members of a family or subfamily, to the use of hidden Markov models that describe the conservation of residues over entire domains or whole proteins. To increase their value as protein classification tools, protein signatures from these 11 databases have been combined into one, powerful annotation tool: the InterPro database (http://www.ebi.ac.uk/interpro/) (Hunter et al., Nucleic Acids Res 37:D211-D215, 2009). InterPro is an open-source protein resource used for the automatic annotation of proteins, and is scalable to the analysis of entire new genomes through the use of a downloadable version of InterProScan, which can be incorporated into an existing local pipeline. InterPro provides structural information from PDB (Kouranov et al., Nucleic Acids Res 34:D302-D305, 2006), its classification in CATH (Cuff et al., Nucleic Acids Res 37:D310-D314, 2009) and SCOP (Andreeva et al., Nucleic Acids Res 36:D419-D425, 2008), as well as homology models from ModBase (Pieper et al., Nucleic Acids Res 37:D347-D354, 2009) and SwissModel (Kiefer et al., Nucleic Acids Res 37:D387-D392, 2009), allowing a direct comparison of the protein signatures with the available structural information. This chapter reviews the signature methods found in the InterPro database, and provides an overview of the InterPro resource itself.