Handling of powders and granular materials is of great importance to industry. However, the knowledge of many powder related processes in industry is poor. In this work, the flow behaviour of powder has been investigated with an advanced tester: the Flexible Wall Biaxial Tester. Flow of powder occurs via the formation of shear planes. For non-cohesive materials one assumes a width of such a shear plane of 10 particle diameters. In this work the Neutron Depolarization technique has been used to investigate the shear region on a microscopic scale. This technique gives direct information of the rotation of the individual particles in the shear region. It appeared that the width of the shear region in cohesive BCR-limestone is much wider than 10 particle diameters. The assumption that the shear region in direct shear testers is a flat plane is incorrect. With the Flexible Wall Biaxial Tester a number of experiments have been performed in order to understand the fundamental flow behaviour of cohesive powder. It has been shown that the flow function is not independent of the intermediate principal stress as is usually assumed. Also anisotropy has a large influence on the behaviour of powder and the measured anisotropic flow function is lower than the regular flow function. These results cannot be explained by the current theory. The reason for these findings is that the particles in the powder form structures that determine the macroscopic behaviour of the powder. This structure is a missing parameter in the current theory. The definition of a structure parameter makes the design of powder with chosen properties possible.