The Data Documentation Initiative (DDI) is an international project to create a standard for information describing statistical and social science data (i.e., metadata). Begun in 1995, the effort brings together data professionals from around the world to develop the standard. The DDI specification, written in XML, provides a format for content, exchange, and preservation of information. Version 3.1 of the DDI standard was released in 2009. DDI fills a need related to the challenge of storing and distributing social science metadata, due to the ubiquity of proprietary file formats and no international standard for the design of codebooks. Version 2 of the DDI standard has been implemented in the Dataverse data repository and the data archives of the Inter-university Consortium for Political and Social Research.