A simple time-independent, mathematical model is proposed for the monotonic and cyclic behavior of concrete under multiaxial stress conditions. An essential feature of the model is a bounding surface in stress space, which is a function of ϵ\N\dm\da\dx = the maximum strain experienced by the material to the present time. The “yield” surface degenerates into the current stress point. Strain increments dϵ\N\di\dj are considered completely plastic and are computed by superposition of: (1) An isotropic component, proportional to the hydrostatic stress increment; and (2) deviatoric and isotropic components, proportional to the octahedral shear stress increment. The plastic modulus for calculation of the latter strain components is a function of: (1) The distance of the stress point from the bounding surface, measured along the direction of the stress increment dσ\N\di\dj; and (2) ϵ\N\dm\da\dx. This functional dependence of the plastic modulus and the fact that the bounding surface shrinks as ϵ\N\dm\da\dx increases allow realistic modeling of the nonlinear unloading and reloading behavior of concrete.