The measurement of particle size distributions has become increasingly important for the testing and certification of equipment used in size analysis. Requirements for quality control compliance with the ISO 9000 series of quality standards have emphasized the need for test standards. This paper describes currently available NIST Standard Reference Materials(SRMs) for particle size, and discusses their development and production. The development includes selection, blending, sampling, and measurement of powders by different techniques and comparison of those techniques. Although the particle size SRMs includes a variety of glass bead materials, the broadest range of sizes is available in the glass bead materials. The spherical shape, known density and known optical index of the glass materials make them a model material for calibration of instruments for powder measurements. Limitations and precautions in the usage of these SRMs will be discussed, as well as plans for future reference materials.