This work provides a summary of application of low-temperature Whispering Gallery Mode spectroscopy to a study of impurity spins in crystalline solids. The method provides an extreme sensitivity that is capable of detecting dilute spins in the most pure fabricated crystals such as sapphire. The study reveals discovery of a number of new impurities in sapphire, quartz and YSO as well as some unexpected phenomena in these systems. Regimes of quantum strong coupling are demonstrated in a number of crystals.