Phase Change Materials (PCM) offer the possibility to store latent heat at a specified temperature, like a battery stores electric energy of a certain voltage. By applying these PCMs in buildings one can store heat to lower peak temperatures and one can use this heat for time periods that the surrounding temperature decreases. By this way indoor temperatures can be better balanced during conventional day-night cycles, in which an overload of heat exists during the day and a shortage of heat can be expected during the evening. Four boxes have been developed in which concrete slabs with and without PCMs were monitored for multiple seasons.