The National Health Data Dictionary, Version 12, updates the data definitions endorsed as national standards for use by those collecting health data in Australia. Version 12 includes definitions for all the National Minimum Data Sets agreed for mandatory collection and reporting at a national level in Australia's health sector. This version of the Dictionary also includes a significantly expanded number of nationally endorsed data definitions for use by clinicians. This set includes Volumes 1 and 2, version 12 supplement, and supplement CDUse of the Dictionary will help ensure that health data in Australia are uniform and of high quality. The format for data definitions continues to be based on the international standard for defining data elements issued by the International Organisation for Standardization. The Dictionary is also available in a searchable database form on CD and the Knowledgebase - Australia's Health, Community Services and Housing Metadata Registry on the Institute's internet home page (www.aihw.gov.au).