For the particle size distribution (PSD) characterization of dry powders various analytical technologies are commercially available. Most of these techniques require the sample to be dispersed, and for pharmaceutical powders liquid dispersion media are often preferred over dry dispersion. In practise, incomplete dispersion and/or dispersion instability are readily observed, potentially leading to a limited accuracy and precision of the method. It is reasonable to argue that it is not so much the size distribution measurement itself but the sample dispersion which is a far more critical aspect in the overall process. If one aims to develop a robust PSD test method, a basic knowledge of the underlying fundamentals is indispensable. For this purpose, and in order to acquaint the reader with a better conceptual understanding of this matter, the wet dispersion of a single component dry powder is discussed in this paper from a theoretical perspective in terms of its cohesion. wetting, rupture and stabilization. To allow a qualitative evaluation of the cohesiveness of a powder, the magnitude of the attractive Van der Waals force relative to liquid bridging and gravity force is discussed. Moreover, attention is paid to the impact of particle deformation, surface roughness and vapour adsorption. In the section on wetting attention is paid to (non-)aqueous dispersion media and the application of suitable wetting agents. Concerning particle disruption, this paper focuses on aspects which induce a lowering of the interparticulate forces of attraction. In addition, active disruption by means of agitation and sonication is discussed. In a further section the paper discusses the parameters which affect the stability of the dispersed particles including the use of enhancing surfactants. The paper concludes by giving some first-hand recommendations to assess the wet dispersion. (c) 2009 Elsevier B.V. All rights reserved.