The following topics are covered: Role of micromechanistic, macroanalytical, and ad hoc testing approaches in rheological studies, and limitations of each. Concept of idealized materials and their rheological properties, equations of state, and constitutive equations. Review of thermodynamic equations of state for idealized gases. Viewpoints on explicit constitutive equations for defining macrorheological properties of solid materials and their limitations, considering localized and stress-history effects. Differential equations for defining constitutive relations and their corresponding mechanical models. Linear models, localized deformation, flow, and relaxation effects. Relation of experimental time scale to relaxation times (internal time scale) of materials in interpreting rheological observations. Role of nonlinear models. Limitation of idealized representations as an engineering tool for property studies on real materials.