Cement-based materials are by far the most important building materials. Microstructure study of cement-based materials has aroused a vast concern all over the world. This book discusses three issues of microstructure study: 1) the hydration process monitoring of cement pastes by this measurement, 2) pore size distribution characterization of cement pastes by the non-contact impedance measurement, and 3) the development of fractal permeability model. The innovative measurement developed in this study modulates the frequency domain and measures both real and imaginary parts of the impedance with non-contact nature. The theoretical basis for pore structure characterization is the fractal electrical network, electrical double layers model and frequency dispersion mechanism. The pore structures interpreted by the method have good agreement with the results obtained by other measurements. The fractal permeability model involves in two fractal dimensions, some structural parameters and minimal and maximal pore diameters. The validity of this permeability model is assessed by other permeability approaches and reasonable agreement between two methods is observed.