There is a great need to understand the fundamental rheological properties of particles as they pertain to industry and the environment. Knowledge of powder rheology can permit scale-up of industrial processes, control of soil erosion, etc. Empirical tests that have developed over the year are inadequate to define the basic constitutive relationships of stress, strain, and rate of powders. Shear rheometry experiments have been developed to accurately measure the material functions of particles in their linear viscoelastic region. These data are used to calculate the cohesive strength of particles in the absence of external loads. Well characterized particles in terms of their size, shape and size distribution have been determined. The effects of these variables on the cohesive strength of particles are reported.