A three-dimensional constitutive relationship for concrete is developed and results are compared to experimental data available in the literature. The model uses the concept of equivalent uniaxial strain, the nonlinear stress-strain equation of Saenz, and the Argyris failure surface for concrete strength. The technique of evaluating the parameters to adjust the constitutive relationship for various concretes is examined. Comparison is made with the biaxial experimental data of Kupfer, Hilsdorf, and Rusch, and with the three-dimensional experimental data of Schickert and Winkler. The model appears capable of simulating the stress-strain response of concrete under a large range of conditions and is easily incorporated into finite element programs. Flow charts for implementation of the constitutive relationship are presented.