6th European Workshop on Structural Health Monitoring. Dresden. Germany. 2012. Report. The goal of Model Order Reduction (MOR) is to catch a model of order lower than that of the real model satisfactory for the purpose of the analysis. The reduced order model should be characterized by a low computational effort but also to be able to estimate the input-output map of the original system in an important region of the input space. Actually, since only a subset of the input space is of interest, this matching should occur in this subset of the input space. This contribution emphasizes some consequences of the adoption of a reduced order model when structural monitoring applications are pursued.