Optical sensing involves an optical measurement that can be related to a desired sample property. The ultimate objective might be the detection of a certain pathogen, the concentration of a harmful gas, the amount of contamination, the presence of a particular DNA sequence, the list is almost endless. The electric field of the probe light interacts with the sample material to yield the signal and the greater this electric field, the greater the signal. Techniques for magnification of an existing electric field are therefore of great interest and importance, and a particularly useful method involves an optical resonator. The resonator is usually a tuned cavity within which interference is arranged constructively to reinforce the field, but not all cavities are useful in sensing because the sample must be introduced into the high-field region. This requirement has led to many different and ingenious resonator configurations. Some have been used for many years. Others are the subject of current research. Nevertheless, they share common fundamental principles.