Soils are composed of grains but they are generally treated as continua in the classical framework of geomechanics. Their macroscopic response under loading, such as their non-linearity, yielding and anisotropy, is controlled by their micro-structure, the characteristics of the grains and the disposition of contacts between them. There have been rapid advances in technology both to investigate the microscopic properties of soils, and to simulate their granular behaviour explicitly through Discrete Element Method (DEM). DEM was originally used to reproduce element tests, but it is now being advocated for boundary-value problems.