Solid dosage forms consist of powders subdivided into a range of different particle sizes. Indeed, each component in the solid state would have different characteristics that must be considered with respect to behavior during manufacture and with respect to drug delivery. Particle size distribution, the measure of subdivision of a solid, can be directly correlated with uniformity of quality and performance. Primary particle size is a physicochemical property that may impact temporal chemical and physical effects such as stability and aggregation. In addition, particle size influences the motion of the powder or suspension that may be required during processing or as an endpoint of the dosage form performance. Descriptions of particle sizeParticle sizes are based on assumptions about the dimension being measured. Moreover, since particles in a finely divided solid exist in a range of sizes, assumptions are made about their distribution and mathematical functions that best approximate the materials under scrutiny. It is important to acknowledge the assumptions and their limitations in accurately describing a particle sizeParticle sizes distributionSize distributions. Comparing the definition adopted with the process or performance characteristic desired maximizes the likelihood of setting relevant product specifications.