We analyzed 99 ERS SAR images of the Colli Albani volcano using the Permanent Scatterers technique. We retrieved the average surface displacement velocities for over 100,000 scatterers for the period 1993--2000, showing that present deformation is concentrated in the area of most recent volcanic activity, with rates up to 2.6 mm/yr in the satellite line of sight. We inverted the observed deformation using several analytical solutions for volcanic sources in an elastic half space. The best fit is obtained with two Mogi sources aligned in a N-S direction at 5 and 7 km depth beneath the western flank of the volcano.