A major part of the infrastructure is constructed of Portland cement concrete. Nevertheless, many aspects of this material are still not well understood. The main constituent of hydrated Portland cement is a hydrous gel, with significant features on the nanometer scale, that tends to be unstable under vacuum. It is difficult to investigate with conventional instrumental methods. Neutron scattering methods possess unique capabilities to characterize cementitious materials and thus to resolve a number of long-standing issues. Among the applications, quasi-elastic neutron scattering is used to follow non-destructively the progress of the main hydration reaction and to determine the distribution of the water content among the various states: i.e. free, chemically bound, or physically trapped in the gel network. Quasi-elastic neutron scattering can also be applied to investigate the process of ice formation in the pore spaces. Neutron diffraction has been used to observe the development of hydrous phases as a function of time. Small-angle neutron scattering (SANS) proves to be well-suited to the characterization of the microstructure. This method is being used to determine the surface and volume fractal dimensions of the gel as a function of time and as a function of additives such as fly ash or silica fume. In addition, SANS has been applied to characterize silica aggregates for the potential for deleterious reactions in highly alkaline concrete. Aside from their application to specific problems, the combination of neutron scattering techniques is leading to new interpretation of the fundamental structure of the Portland cement gel.