This is DDI-Codebook version 2.5, published on 2012-01-17 as approved for publication by the DDI Alliance Expert Committee, with minor revisions. The current publication includes the schemas, field-level documentation publication in HTML, and change documentation. The continued development of DDI-Codebook reflects the work of the committees and individuals who developed DDI versions 1.0 - 2.1 as well as the collective ideas, needs, and work of the Expert Committee of the DDI Alliance. We would like to thank the following member organizations which have supported the DDI Alliance in their development of DDI version 2.5: University of Alberta, Canada Australian Bureau of Statistics (ABS) Australian Data Archive (ADA) University of California, Berkeley -- Computer-Assisted Survey Methods Program and UCDATA, USA Centre for Longitudinal Studies, Institute of Education University of London (Associate Member) Centro De Investigaciones Sociologicas (CIS), Spain Cornell University (CISER) Danish Data Archive Data Archiving and Networked Services (DANS), The Netherlands Finnish Social Science Data Archive German Socio-Economic Panel Study (SOEP) GESIS - Leibniz Institute for the Social Sciences University of Guelph, Canada Institute for Qualitative Social Science (IQSS) at Harvard University Institute for the Study of Labor (IZA), Germany Institute for Social and Economic Research (ISER) International Association of Social Science Information Service and Technology (IASSIST) International Federation of Data Organizations (IFDO) Inter-university Consortium for Political and Social Research (ICPSR), USA Massachusetts Institute of Technology (MIT), USA Minnesota Population Center (MPC), USA Norwegian Social Science Data Service (NSD) Open Data Foundation, USA Princeton University, USA Research Data Centre of the German Federal Employment Agency, Institute for Employment Research (IAB) Roper Center, USA Stanford University, USA Statistics New Zealand Survey Research Operations, University of Michigan Swedish Social Science Data Service (SSD) Swiss Data Archive for the Social Sciences (SIDOS) University of Toronto Scholars Portal United Kingdom Data Archive University of Washington, Center for Studies in Demography & Ecology (CSDE), USA United States Bureau of Labor Statistics (Associate Member) World Bank, Development Data Group (DECDG) DDI 2.5 XML Schema is released under the terms of the GNU Lesser General Public License. Details see the file license.txt.