The usual theoretical approach to analyzing the microstructure and properties of composite materials [9,10], where the matrix is a known quantity, is to first examine the dilute limit, where the inclusion phase is at a such a low volume fraction that each particle can be considered separately from all the others. One then builds up the full composite and examines the effect of having many inclusions. Later in this chapter a multi-scale approach to modelling the microstructure of concrete [11] will be studied, in which, analogous to the composite approach given above, one first develops models on different length scales and then links such models together to try to understand the full multi-scale problem. Consistent with this approach, the microstructure of the ITZ can be considered at two levels: the formation of microstructure near a single aggregate particle surface, and the formation of a global network of ITZ microstructure throughout the concrete.