The response of many geotechnical problems is governed by the development of discontinuities within the soil mass. Model tests, at one gravity and on a centrifuge at 100 gravities, on granular soils of different particle sizes, have been used to study the formation of discontinuities over a moving fault and in front of a rotating wall or blade. Discontinuities are observed as zones of localised shearing deformation and associated density decrease. The pattern of discontinuities is controlled by the changing dilation of the soil, and varies during the course of a test. The pattern of discontinuities is linked with particle size : in tests conducted at the same stress level, the same pattern of discontinuities is observed if the ratio of boundary movement to particle size is the same. Dilatancy and friction in granular soils are affected by stress level. Stress levels can be correctly reproduced in small models tested on a centrifuge and observations of continuous soil response can then be extrapolated from model to prototype scale. Kinematic effects associated with discontinuous soil masses require statements about displacements as well as strains, and care is required in predicting large scale response on the basis of the results of small model tests.