Summary: With the von Mises yield criterion as basis, a generalised isotropic yield function for incompressible yielding has been formulated. The proposed yield function includes as limiting forms, both the classical yield criterion of Tresca and that of von Mises. It is shown that the general yield criterion includes a twelve-sided, piecewise linear condition of yielding. A method of relaxing the restrictions imposed by the vertices associated with a piecewise yield criteria is described. The formulation of a composite yield condition, comprising a piecewise, linear and continuous, yield function, together with a continuously differentiable form of yield function, is described. The use of the proposed yield criterion is reviewed in the context of the two different types of experiment for determining the yield condition satisfied by a given material.