This paper summarizes the basic formulation for the inelastic analysis which the author and his colleagues have been pursuing for several years and have led to a general purpose nonlinear continuum mechanics program COMPOSITE-III. Mechanical model approach is adopted which allows for behaviors of both time independent plasticity and time dependent viscoelasticity. This model also facilitates the inclusion of a variety of internal variables proposed recently in the literature on constitutive equations. Derivation of the incremental constitutive relation from the model is shown for incorporation into the standard finite element solution procedure. Correlation of the derived equation with those from other existing theories as well as the experimental determination of characteristic constants or parameters of the relevant model are discussed. Although the present paper concerns itself principally with the material nonlinear behaviors, a brief review is given to the method of solution for problems with geometric nonlinearities. Numerical examples from the file of our recent computations are the elastic-plastic-creep analysis of a shear lag test specimen and occurrence of diffuse neck instability in plane stress, plane strain and axisymmetric tensile test specimens.