The main purpose of this work is the validation of the model of the Ceira Viaduct with the calculation of the modal parameters and, in particular, with the comparison between numerical and experimental results. To do this, the most important aspects of modal are initially presented. The contents are presented providing, at first, a theoretical background of the topic. The theoretical aspects are addressed using the classical approach commonly used for dynamic structural analysis, by posing particular attention on the aspects then considered in some applications. The examples presented are implemented by the software MATLAB and Autodesk Robot Structural Analysis. In some cases the results obtained can be compared to some analytical results, this allows the validation of the implement routines.