This chapter deals with the effects of water on the mechanical behaviour of pavements. The analysis is based on constitutive considerations. Constitutive models devoted to both routine and advanced pavement analysis and design are introduced and both the resilient behaviour as well as the long term elasto-plastic approaches are presented. As soon as the approach considers the material as a two phase (solid matrix and a fluid), the introduction of the effective stress concept is required. In the last section an analysis is made on the extension of the constitutive models to the characterisation of partially saturated materials.