Designing multicell cavities with optimal electromagnetic characteristics is a challenging engineering task, requiring the modal analysis of an immense number of often only slightly different cavity geometries and resulting in an enormous effort. Perturbative methods enable geometric parameter studies based on the eigenmodes of only one initial (unperturbed) geometry. They base on the concept to expand the eigenmodes of a modified (perturbed) geometry in terms of the eigenmodes of the unperturbed geometry and thus allow for an enormous reduction of the computational effort.