The use of parameterized macromodels has become more and more popular in a wide range of applications, from Electromagnetic Compatibility to microwave engineering, where they can be employed for the analysis, design and optimization of microwave structures. It is quite common, in fact, that for design optimization purposes or sensitivity analysis some geometric properties and physical quantities are left undefined and become variables of the system. Usually data upon which the macromodel is built come from first-principle solvers. This implies that, in order to simulate all configurations that are necessary to build the model, an extremely large amount of full-wave analyses must be performed.