The author examines a simple extension, to the nonlinear case, of memory-type theory based on the Boltzmann-Volterra superposition principle. It is shown that given certain assumptions the quasi-linear theory of viscoelasticity reduces to introduction into the equations of linear memory theory of a single stress- or strain-intensity function. This function is determined from creep or relaxation tests. A successive-approximation method is presented for solving problems of nonlinear viscoelasticity with the aid of the equations introduced. It is shown that in the case of simple loading the equations of the theory of small elastic-plastic deformations are an analog of the equations considered.