The central theory of critical state soil mechanics is the critical state theory. It states that any soil under sustained uniform shearing will reach a state called the critical state. The theory states that there is a unique relationship between the shear strength, the mean pressure and the element volume at critical state, and this relationship can be predicted. The chapter illustrates the stress-strain relationship of loosely and densely packed granular systems confined at an initial confining pressure and then sheared. There is experimental evidence of the existence of the critical state for clays. The chapter shows the final stress and strain state for a series of triaxial compression tests of Weald clay for both normally consolidated and overconsolidated specimens. Various normalized state indices are available and are often used in constitutive models. The state parameter measures the distance between the current void ratio and the critical state void ratio.