Abstract The sections in this article are Definitions and Generalities Particles Particle Size Nuclearity of Particles Dispersion Relationships Between Particle Size, Surface Area and Dispersion Methods of Particle Size Measurement Particle Size Measurements by Gas Chemisorption Introduction and Principles Gas Adsorption–Desorption Methods A Static Methods B Dynamic Methods C Desorption Methods Choice of Adsorbate Gases Hydrogen Chemisorption; HydrogenOxygen Titration Conclusion and Recommendations X ‐Ray Diffraction Line Broadening Analysis Introduction Elementary Line Broadening Analysis: the Scherrer Equation Complete Line Profile Analysis: the W arren– A verbach Method Examples of Application of Classical LBA to Catalysts Whole‐Powder Pattern Fitting Methods: the Rietveld Refinement Limits of Application of Methods Conclusion Small‐Angle X ‐Ray Scattering Analysis Introduction Specific Surface Area Measurements: P orod's Law Mean Size Parameters: Guinier Radius Calculation of Size Distribution Application of SAXS to Metal Particle Size Determination in Supported Catalysts Anomalous Small‐Angle X ‐Ray Scattering ( ASAXS ) Conclusion Determination of Particle Size by Electron Microscopy Scope Fundamentals of Imaging and Contrast A Transmission Electron Microscope B Scanning Transmission Electron Microscope C Scanning Electron Microscope Preparation of Samples for TEM and STEM Examination Particle Size Measurement Selected Examples of Particle Size Distribution Selected Examples of 3D Localization of Particles in Supports by Electron Tomography Particle Size Measurements by Magnetic Methods