As part of the Terrafirma Validation Experiment a comparison of Persistent Scatterer Interferometry (PSI) with in-situ measurements is performed. The experiment purports to validate the processing chains of partners participating in the Terrafirma project, in order to obtain a better understanding of the performance of the techniques. Two test sites are chosen in the Netherlands: (i) the trajectory of a subway line in the centre of Amsterdam, still to be constructed, and (ii) the area around the city of Alkmaar, affected by subsidence due to gas extraction. The Amsterdam site is monitored using Envisat-ASAR data, whereas for the Alkmaar area both ERS1/2-AMI and Envisat-ASAR data sets are used. Due to the difference in deformation phenomena, available ground truth and surface characteristics, different validation procedures are applied to the two test sites. Here we describe these validation methodologies and the validation results.