Surface plasmon resonance, ellipsometry, and the quartz crystal microbalance are surface-sensitive techniques that provide valuable information of optical and mechanical properties of interfaces. They all are considered to be macroscopic techniques in the sense that they supply global parameters-such as refractive index, thickness, or surface stiffness-as average magnitudes over relatively large sample areas. Another feature shared by all these techniques is that they all rely on appropriate mathematical models to get meaningful values of surface parameters. However, in many cases, choosing the right model is not straightforward.