Two methods to define mean particle diameters exist, viz., the statistics based ISO/DIN method and the physics based Moment-Ratio (M-R) method. The ISO/DIN method characterizes a size distribution by an average particle diameter of that distribution. The M-R method expresses mean diameters as ratios of two moments of a size distribution. The M-R method enables the user to derive mathematically or select empirically the type of mean diameter from the physical product or process property to be described by that mean diameter, irrespective the shape of the size distribution. The application of both definition methods to theoretical derivation and to empirical selection of that type of mean diameter is evaluated. The statistical character of the ISO/DIN system hampers its application to theoretical derivation and empirical selection of mean particle diameter types describing product or process properties. Although the ways of defining mean particle diameters according to both methods differ mathematically, a clear mathematical relationship exists.