The sensitivity analysis in the framework of the finite element method requires the calculation of the derivatives of the system matrices with respect to the parameters of interest. When the sensitivity with respect to geometric modifications is investigated, the problem becomes very complicated due to two factors. First, the geometry is represented by a very large number of parameters, the position of the nodes of the FE mesh, which cannot be considered explicitly in a sensitivity analysis. Second, the system matrices do not depend explicitly on the nodes’ position; thus, the calculation of the derivatives needs a proper formulation. This chapter addresses these two problems using a modal parametrization of the geometric variations and exploiting the use of the directional derivatives to represent the sensitivity of the system matrices. The proposed method is illustrated through a simple two-dimensional example and validated using the finite differences method.