The Protein Circular Dichroism Data Bank is a deposition and user-friendly archive of circular dichroism (CD) and synchrotron radiation circular dichroism (SRCD) spectra, with associated software for validation, analysis and searching. It aims to provide a publicly-accessible resource for structural biology, biotechnology and bioinformatics that enables data sharing and data mining of validated CD data. It contains both spectral data and its associated metadata describing the sample and experimental conditions, with links to sequence and crystallographic data bases. It is located online at: http://pcddb.cryst.bbk.ac.uk. Its functions, contents, structure and features are described in detail in Wallace et al., 2006 [1] and Whitmore et al., 2006 [2].