Improved methods of pavement design require that soil strength should be characterised in terms of fundamental parameters. Although the strength parameter CBR widely used in present methods of road design is empirical in origin, it is shown that it can be related to the undrained shear strength of cohesive soils by one of two linear relationships, one characterising soils in an undisturbed overconsolidated state and the other soils that are remoulded. After validating these relationships it is shown that a penetrometer with a soil can be used to estimate its CBR. The penetrometer can then be used to characterise soil strength in terms of other shear strength or CBR