The effective properties of a two-phase composite depends both on the constituent materials and the microstructure. Starting from David J. Bergman’s work on the integral representation formula (IRF) approach for deriving bounds on dielectric properties of composites with isotropic constituents, followed by Golden & Papanicolaou’s work on establishing the link between IRF and spectral theory, the IRF is on now a firm mathematical background for further generalization from isotropic constituents to general constituents. The key feature we are looking for is the separation of information on material contrast from that of microstructure in the IRF. This is especially important for (visco)elastic composites because their current available IRFs do not have this feature, even when constituents are isotropic. In this talk, we will present a recently derived IRF for general (visco)elastic composites with the above feature. The relation between IRF and the quantification of microstructure (as spectrum of a generalized Helmholtz projection operator) will be explained. If time permits, numerical results will be presented and the connection between the IRF and the fabric tensor introduced by Stephen C. Cowin will also be discussed.