The photometry of retroreflectors involves special techniques and accurate and reproducible results are difficult to achieve. The photometric performance of a retroreflector depends critically on the measurement geometry and the angular apertures of the illuminating source and the photometer. To simulate practical use conditions, very small angles are involved which need precise adjustment and which limit the amount of light available for measurement, so that a sensitive photometer is needed. Precautions are necessary to avoid errors due to stray light. Many retroreflectors are coloured so that the spectral power distribution of the illuminating source must be specified and careful attention paid to the adaptation of the photometer to the CIE standard photometric observer. Work is in progress internationally which it is hoped will lead to recommendations for a measurement technique which will reduce the present discrepancies in results between laboratories to an acceptable level.