Particle breakage explains the specific features of rockfill behaviour. Relevant features concern the dilatant behaviour under increasing confining stress, the long term deformations (creep) and the response under changes in relative humidity (RH). RH effects on deformation are well known in practice and result in marked collapse behaviour of rockfill structures as a consequence of full or partial saturation. The lecture will first present an ordered set of experimental results. The following aspects will be introduced: Suction effects on compressibility and yielding of compacted gravel specimens of different nature and grain size distribution, delayed creep deformations under isotropic conditions, deviatoric behaviour, dilatancy and the evolution of grain size distribution. These results sets the ground for the presentation and discussion of a Distinct Element model developed in an attempt to create a “numerical” laboratory capable of predicting real behaviour. The model includes particle shapes which reproduce, in a reasonable manner, the irregular shape of real rock fragments. Particle breakage is introduced as a fundamental deformation mechanism. Unlike other approximations reported in the literature, particle breakage was approached from the perspective offered by fracture mechanics. Particle breakage is the consequence of the propagation of pre-existing cracks. Crack propagation velocity of each individual particle is approximated by means of analytical solutions. Delayed deformations are a natural consequence of the model and the effect of RH can also be introduced in a simple manner. The process of parameter determination will be discussed with specific reference to large diameter oedometer and triaxial tests.