Nowadays, chemical sensors and biosensors play a crucial part in everyday life. In the wide field of sensors, those based on waveguide-sensing principles are used, for example, for routine analyses, and more generally, for many applications in physics, chemistry, biology, or medical and pharmaceutical tasks [1—3]. The versatility of waveguide-based sensors combined with their high sensitivity has paved the way for such sensor platforms to play a role in the wide range of possible applications — with many more waiting to be explored. Yet, one crucial prerequi- site for the success of a technology is sensitivity, and the sensitivity of waveguides is directly linked to their refractive index. Due to this, waveguide materials with a high refractive-index have attracted recent attention towards the development of new sensors. In addition to the refractive index of the waveguide material, the so- called evanescent field can be optimized to reach high sensor sensitivity. The evanescent field is the exponentially decaying part of the guided light wave lying outside the waveguide. Evanescent field sensors, as presented in this chapter, use this effect to measure the interaction between the sample and the evanescent field. Such sensors can operate either in a label-free manner, i.e., they detect the analyte directly without any reporter molecule, or the evanescent field is used to excite fluorophores attached to the analyte, close to the waveguide surface. The advantage of the selective excitation of bound fluorophores within the evanescent field is that the background signal caused by unbound fluorophores is reduced to a large extent.