Particle Size is a notion introduced for comparing dimensions of solid particles (flecks), liquid particles (droplets), or gaseous particles (bubbles). The notion of particle size applies to *Colloidal particles; *Particles in ecology; *Particles present in granular material whether airborne or not; *Particles that form a granular material. The particle size of a spherical object can be unambiguously and quantitatively defined by its diameter. However, a typical material object is likely to be irregular in shape and non-spherical. The above quantitative definition of particle size cannot be applied to non-spherical particles. There are several ways of extending the above quantitative definition, so that a definition is obtained that also applies to non-spherical particles. Existing definitions are based on replacing a given particle with an imaginary sphere that has one of the properties identical with the particle. *Volume based particle size equals the diameter of the sphere that has same volume as a given particle. Which equals 2*(3*volume(particle)/4/pi)^(1/3). *Weight based particle siz equals the diameter of the sphere that has same weight as a given particle. Which equals 2*(3*weight(particle)/4/pi/density(particle)/g)^(1/3). *Area based particle size equals the diameter of the sphere that has the same surface area as a given particle. Which equals 2*(area(particle)/4/pi)^(1/2). *Hydrodynamic or aerodynamic particle size equals the diameter of the sphere that has the same drag coefficient as a given particle. Another complexity in defining particle sizeappears for particles with sizes below a micrometre. When particle becomes that small, thickness of interface layer becomes comparable with the particle size. As a result, position of the particle surface becomes uncertain. There is convention for placing this imaginary surface at certain position suggested by Gibbs and presented in many books on Interface and Colloid Science. Definition of the particle size for an ensemble (collection) of particles presents another problem. Real systems are practically always polydisperse, which means that the particles in an ensemble have different sizes. The notion of particle size distribution reflects this polydispersity. There is often a need of a certain average particle size for the ensemble of particles. There are several different ways of defining such a particle size. *There is an International Standard on presenting various characteristic particle sizes. This set of various average sizes includes median size, geometric mean size, average size. There are several methods for measuring particle size. Some of them are based on light, other on ultrasound, or electric field, or gravity, or centrifugation. They are briefly described in the section particle size distribution. (An OSH glossary used in safety and health at work which is, adopted by ILO