This review paper aims at giving an overview of the different applications of thermodynamic equilibrium calculations in cementitious systems. They can help us to understand on a chemical level the consequences of different factors such as cement composition, hydration, leaching, or temperature on the composition and the properties of a hydrated cementitious system. Equilibrium calculations have been used successfully to compute the stable phase assemblages based on the solution composition as well as to model the stable phase assemblage in completely hydrated cements and thus to asses the influence of the chemical composition on the hydrate assemblage. Thermodynamic calculations can also, in combination with a dissolution model, be used to follow the changes during hydration or, in combination with transport models, to calculate the interactions of cementitious systems with the environment. In all these quite different applications, thermodynamic equilibrium calculations have been a valuable addition to experimental studies deepening our understanding of the processes that govern cementitious systems and interpreting experimental observations. It should be carried in mind that precipitation and dissolution processes can be slow so that thermodynamic equilibrium may not be reached; an approach that couples thermodynamics and kinetics would be preferable. However, as many of the kinetic data are not (yet) available, it is important to verify the results of thermodynamic calculations with appropriate experiments. Thermodynamic equilibrium calculations in its different forms have been applied mainly to Portland cement systems. The approach, however, is equally valid for blended systems or for cementitious systems based on supplementary cementitious materials and is expected to further the development of new cementitious materials and blends.