Failures often originate at the interfaces in the packaging of mechatronic devices because of the differences in the thermo-mechanical properties of the materials in contact. To enhance the reliability of these devices, it is necessary to have non-destructive analysis techniques such as spectroscopic ellipsometry (SE) to probe the quality of the surfaces and interfaces when the environmental constraints vary. In the field of characterization spectroscopy, SE has become indispensable in microelectronics, as well as in the study of semiconductors, protective coatings based on polymers, metals or other types of metamaterials. It is used to characterize thin films, mono- or multi-layers and bulk materials from a structural and optical point of view by probing transitions, whether electronic, vibrational or rotational. In the ultraviolet (UV)-visible and near- to mid-infrared (IR) ranges, which correspond, respectively, to electronic and vibrational absorption, SE reveals the composition, structure (amorphous or crystalline), porosity and morphology (density, roughness, etc.) of materials as a function of the light wavelength. In order to be used, SE parameters generally require an inverse method based on the simplex, the Levenberg-Marquardt or the Broyden-Fletcher-Goldfarb-Shanno algorithm to determine the dielectric function or complex optical constants. This chapter describes the SE technique and illustrates its application with two examples of characterization, that of sintered silver and polymers present in a mechatronic device. A study of the effects of temperature in dry and wet conditions is also presented and discussed in terms of optical properties.