Microscopy has played an essential part in developing our knowledge of Portland cement clinker phase composition, and is routinely used to monitor kiln operating conditions to insure cement quality. Our developing understanding of the phase composition of clinker coincides with the early days of the polarized light microscope where the then-new instrument was used to resolve conflicting theories on the phase constituents of cement clinker, essentially a man-made rock formed through sintering a ground, blended mixture of limestone, clay, and iron oxides. The phase abundance, distribution and texture of cement clinker reflect the combination of proportioning, grinding, and homogenization of the raw materials, and the firing and cooling history of the clinkering process. The ability to visualize, record and quantify phase compositional and textural attributes of the clinker allowed cement chemists a view into this process, to develop a better understanding on clinker production, and to be able to identify problems in the preparation and firing of the raw materials for improved production of the clinker. Today, most microscopy uses polished sections of clinker and reflected light, and quantitative methods include the point-count analysis, as well as image processing and analysis. The development of certified clinker reference materials have facilitated the development of the first standard test methods for clinker microscopy and X-ray powder diffraction. The application of the scanning electron microscope (SEM) allows analysis of the fine-grained, multi-phase particles of hydraulic cements and pozzolans, expanded our view into their mineralogical and textural characterization. Modeling the hydration process has developed to the point where selected properties of cement performance may be predicted from a well-characterized cement through microscopy. Cement phase mineralogy and textural characteristics, captured through SEM imaging, and particle shape characteristics quantified through X-ray tomography has allowed the generation of 3D virtual cement particles that retain the phase and textural attributes, providing realistic inputs to cement hydration models.