The purpose of this report is to update and replace CIE 54-1982 Retroreflection: Definition and Measurement. This report is divided into nine main sections. These begin with chapter 2 which is concerned with general definitions of retroreflection, while chapter 3 defines geometrical terms. Chapter 4 combines their collection into four geometric systems applicable for different elements of the retroreflection phenomena. Chapter 5 defines the photometric terminology as did the original publication 54, but defines the photometric terms with reference to spectroradiometric concepts as is the current international standards procedure. The corresponding laboratory calibration and measurement techniques are described in the next section, chapter 6, together with basic instrument requirements and traceability to national metrology laboratories. To relate the driver's geometry to the four systems, vector notation and a series of typical examples are given in chapter 7. This approach will aid the transportation community in accessing the necessary discrete angles to assure procurement compliance and the field service life of retroreflective devices and materials. An important section in the original publication 54, dealing with the colorimetry of retroreflectors, is expanded in chapter 8 to include new instrumentation presently available and to recommend standard test geometries for both daytime and nighttime measurements. A new section, chapter 9, describes general field instruments available for in-situ measurements of retroreflectors. Included in this section is a guide for manufacturers to use in their specifications, listing important parameters of the instruments which can be a part of the data sheet. Lastly, chapter 10 addresses the computer format for retroreflectance data in order to allow measurement results to be exchanged world-wide. Four appendices complete the report with all the transformation equations between the four systems with examples, a list of keywords, data format examples and a treatise on aperture synthesis.