The setting and hardening process of concrete can be considered as the most critical time period during the life of a concrete structure. To assure high quality and avoid problems in performance throughout the life of the material, it is essential to have reliable information about the early age properties of the concrete. This paper presents a new method to monitor the hydration process of cementitious materials at early ages. The technique is based on the measurement of the reflection coefficient (or reflection loss) of high frequency shear waves at an interface between a steel plate and e.g. concrete. Several experimental studies that examine the ability of the wave reflection method to qualitatively and quantitatively describe the setting and hardening process of cement paste, mortar and concrete are presented. The results show that parameters such as setting time, compressive strength, elastic and visco-elastic moduli and degree of hydration of cementitious materials can reliably monitored with the wave reflection method. The relationship between the measured reflection loss and the compressive strength was successfully used in a first field trial of the method to determine the inplace strength of precast concrete elements.