An introduction to the constitutive modelling of unsaturated soils is presented. To facilitate understanding by the inexperienced reader, the paper considers the basic aspects of modelling of the behaviour of unsaturated soils from a qualitative point of view. First, the stress framework is discussed by analysing the various possible stress and strain variable combinations. The main concepts of a constitutive model for unsaturated soils using Bishop’s generalised effective stress are then presented. The constitutive modelling uses the Cam-Clay framework. The conception of an unsaturated model is linked to the experimental analysis of a soil response to typical hydraulic and mechanical loading cycles.