A back analysis method for geotechnical problems has been proposed on the basis of the Virtual Work Principle, in which the original structural model is compared through an iterative process to a modified model with prescribed field measurements. This method is very robust, stable, effective, and works also for non linear rock mass behaviour. For illustration, this method has been applied to a subsidence problem in a mining area, to a shallow tunnel numerical model, and to a tunnel excavation case history.