Soils are particulate materials. Therefore, the behavior of soils is determined by the forces particles experience. These include forces due to boundary loads (transmitted through the skeleton), particle-level forces (gravitational, buoyant, and hydrodynamic), and contact level forces (capillary, electrical and cementationreactive). The relative balance between these forces permits identifying various domains of soil behavior. Furthermore, the evolution of particle forces helps explain phenomena related to unsaturation, differences between drained and undrained strength under various loading modes (including the effect of plasticity), sampling disturbance, and fines migration during seepage. Generally accepted concepts gain new clarity when re-interpreted at the level of particle forces.