Particle interactions underpin the performance of pharmaceutical powders during processing and manufacture. The behavior of particles in bulk dictates their behavior. The forces of interaction between particles influence their flow and dispersion and the ease with which they can be suspended in any medium. These interactions are in the form of electrostatic and capillary forces that are reciprocally but not linearly related, mechanical interlocking arising from surface asperities, and bulk van der Waals forcesVan Der Waals forces. Variations in these interactions can lead to poor dose uniformity and biopharmaceutical properties. Pharmaceutical inhalation aerosols are a unique example of the way interparticulate forces influence particle behavior and illustrate the considerations that need to be given to these forces.