Three examples were considered of the use of high resolution Fourier-transform optical spectroscopy for studying quality of rare-earth-doped crystals. The first example was connected with defects present in crystals grown by flux techniques. The second example dealt with detection of stresses and deformations in rare-earth-containing crystals, by registering splitting of spectral lines. The third example showed that a very small amount(at the level of ppm) of different RE ions present in a crystal could be determined using high-resolution spectroscopy. This work was carried out by the author’s group in collaboration with several institutions in Russia and abroad. Three examples were considered the use of high resolution Fourier-transform optical spectroscopy for studying quality of rare-earth-doped crystals. The first example was connected with defects present in crystals grown by flux techniques. The second example dealt with detection of stresses and deformations in rare-earth-containing crystals, by registering splitting of spectral lines. The third example shows that a very small amount (at the level of ppm) of different RE ions present in a crystal could be determined using high-resolution spectroscopy. work was carried out by the author’s group in collaboration with several institutions in Russia and abroad.