For over 70 years the International Commission on Illumination (CIE) has provided recommendations about the precise way in which the basic principles of colour measurement should be applied. CIE 15:2004 Colorimetry represents the latest edition of these recommendations and contains information on standard illuminants; standard colorimetric observers; the reference standard for reflectance; illuminating and viewing conditions; the calculation of tristimulus values, chromaticity coordinates, colour spaces and colour differences; and various other colorimetric practices and formulae. This publication is consistent with the fundamental data and procedures described in the CIE Standards on Colorimetry. This publication, which replaces CIE 15.2 (and is not to be known as 15.3!), includes details of the CIE DE2000 colour difference equation; spectral power distributions for sets of halophosphate lamps, DeLuxe type lamps, three-band lamps, multi-band lamps, high pressure sodium lamps and high pressure metal halide lamps. The nomenclature for the recommended geometries has changed, and there are even changes to the equations defining the parameters of the CIELAB colour space! The publication is accompanied by CD-ROM that contains all the tables of standard and recommended spectral distributions and a program (for Windows operation systems) to perform interpolation of spectra related to reflection or absorption measurements. Some of these colorimetric tables are also free available to download under Selected Colorimetric Tables. Thus this publication represents the colorimetric state-of-the-art and should find a place on the bookshelf of every colour scientist.