  Optical whispering gallery mode (WGM) microresonators, confining resonant
photons in a microscale resonator for long periods of time, could strongly
enhance light-matter interaction, making it an ideal platform for all kinds of
sensors. In this paper, an overview of optical sensors based on WGM
microresonators is comprehensively summarized. First, the fundamental sensing
mechanisms as well as several recently developed enhanced sensing techniques
are introduced. Then, different types of WGM structures for sensing as well as
microfluidics techniques are summarized. Furthermore, several important sensing
parameters are discussed. Most importantly, a variety of WGM sensing
applications are reviewed, including both traditional matter sensing and field
sensing. Last, we give a brief summary and perspective of the WGM sensors and
their role in future applications.
