Dry granular materials, such as sand, sugar and powders, can be poured into a container like a liquid and can also form a pile, resisting gravity like a solid, which is why they can be regarded as a fourth state of matter, neither solid nor liquid. This text focuses on defining the physics of dry granular media in a systematic way, providing a collection of articles written by recognised experts. The physics of this field is full of challenges, but many questions (such as kinetic theories, plasticity, continuum and discrete modelling) also require the participation of mechanical and chemical engineers, soil mechanists, geologists and astrophysicists. This volume gathers the relevant concepts from all these disciplines and should enable the reader to gain an understanding of the foundations, as well as the open questions, of the physics of granular materials.