Most practitioners in the pharmaceutical, mining, environmental, paints and coatings, and other fields have reason to know the sizes of particles in a given powder or slurry sample. While many technologies exist to describe that collection of sizes, the reports delivered by those technologies are written in their own vocabulary. This particle size lexicon can convey a tremendous amount of information as long as the terms and their definitions are clearly understood and their underlying assumptions are fully considered. This discussion introduces the common values encountered on a typical particle size report and provides references for deeper mathematical interpretations. With this information in hand, the reader will be well-equipped to judge compliance.