Recent works on bismuth germanate and lithium niobate are described with relation to the behavior of impurity or dopant ions and their effect on optical properties. For bismuth germanate crystals grown by Bridgman method the mean segregation coefficients for several elements were found to be very small as compared with those for Czochralski method. Radiation damage experiments revealed that Pb, Mn, Fe and Co have the most deteriorative effect. Whereas Ni, Ga and Mg affect slightly, and Al, Si, Ca and Cu seem to have no measurable effect on radiation damage of bismuth germanate crystals. For lithium niobate absorption edge of congruent crystals doped with Mg0 shows a minimum at some 5 mol% Mg. This threshold effect of Mg dopant finds its origin in its substitution behavior in host lattice and could be closely related to the occupancy (4.58%) of Nb in Li site in congruent crystal. For the first time the crystal structure of lithium niobate heavily doped with Mg0 was investigated directly, resulting in a best model with all the Mg accommodated in Li sublattice. On the basis of experimental data the substitution behavior of Mg could be summarized as the following: With Mg content below the threshold value, replacement of Nb in Li site by Mg is the dominant process, leading to a shift of absorption edge toward shorter wavelengths. As Mg content reaches the threshold value,most of the Nbin Li site are replaced, resulting in the observed minimum of the absorption edge curve. With further increase of Mg content, the substitution of Li by Mg becomes prevailing, causing a slight counter-shift of the absorption edge.