The dynamic behaviour of cultural heritage buildings is quite complex, due to their mechanical and geometrical characteristics. So the structural identification and the seismic analysis are very hard and also the numerical analysis by means of finite element models contains large uncertainties. Therefore, the experimental analysis, often based on output only measurements, could be very important to improve our understanding about their dynamic behaviour. In this paper two case studies are shown, in which the measurements of the seismic response in the first case, and of ambient vibrations in the second one, were used to point out the main structural features and to analyse the vulnerability to seismic actions.