This document provides information for the Pharmaceutical Industry and the Federal Drug Administration (FDA) regarding the selection of suitable particle-size analysis techniques, development and validation of particle-size methods, and the establishment of acceptance criteria for the particle size of drug substances used in oral solid-dosage forms. The document is intended for analysts knowledgeable in the techniques necessary to conduct particle-size characterization (a table of acronyms is provided at the end of the document). It is acknowledged that each drug substance, formulation, and manufacturing process is unique and that multiple techniques and instruments are available to the analyst.