Whispering gallery optical parametric oscillators (WGR OPOs) are monolithic sources for tunable coherent and
non-classical light. They are based on total internal reflection. Since reflection losses are negligible, their oscillation
threshold can be far below one milliwatt. With sub-millimeter diameters, they are the most compact OPOs
demonstrated so far. Recent experimental results demonstrate that WGR OPOs emit coherent light tunable
over hundreds of nanometers. Operation in the visible as well as in the near-infrared has been demonstrated
with up to 30 % conversion efficiency. These results indicate a great potential of WGR OPOs for spectroscopic
and sensing applications.