Microwave remote sensing is the most recent experimental methodology suitable to the non-contact measurement of deflections on large structures, in static or dynamic conditions. After a brief description of the radar measurement system, the paper addresses the application of microwave remote sensing in ambient vibration testing of two historic masonry towers. The study clearly highlights that: (a) the actual displacement of the investigated towers under ambient excitation (wind, micro-tremors) is fairly larger than the radar sensitivity; (b) the deflections are generally quite low, so that only the lower modes can be identified from the deflection time-histories; (c) only few points of the investigated towers were characterized by appropriate electromagnetic reflectivity, allowing accurate measurements of the relevant deflection time-histories.