Synthetic Aperture Radar Interferometry allows the coherent combination of radar images acquired from different viewpoints. The interferogram generated by substracting their phases has got information related to the geometry of the scene, i.e. the topography and small movements in the scene. The interferometric process may he therefore addressed to obtain either a Digital Elevation Model of the observed area or a precise quantitative evaluation of the movement in the scene in the radar looking direction with a precission related to the wavelenght, which is 6.4 cm in the case of the ERS European satellites. In the article we will show the interferometric process for DEM generation and the foundations of differential interferometry for small movements detection and atmospheric propagation studies with ERS examples.