Hydrothermally cured or autoclaved cement-based building products have provided many challenges to the researchers, manufacturers and users since their inception nearly hundred years ago. The advantages, including the development of high strength within a few hours and a reduction of drying shrinkage, of the hydrothermal curing process have resulted in a variety of building products; inevitably, the technology of their production has undergone many stages of refinement. With the advent of non-conventional starting materials for the production of modern cements, and the push to utilise renewable resources to form blended cements, the chemical and physical make up of hydrothermally cured building materials have changed considerably in recent years and will continue to change. It is, therefore, important to understand the chemical reactions taking place in an autoclave, and the consequent phase developments, if building materials produced by this process continue to be successful in the long term. A wide range of analytical techniques exists for characterising the phase development in cement-based materials. The purpose of this paper is to illustrate the strength of thermal methods, especially when used in combination with other analytical techniques, in the understanding of hydrothermal reactions.