For the construction of thin-disc lasers, active media with high RE-dopant concentrations and high thermal conductivity are required. YAG is well known for its excellent thermal properties, which are surpassed only by the sesquioxides SC2O3, Y2O3, and Lu2O3. Among these, LU2O3 is the most promising oxide because of its high Yb-doping capability at nearly no drop of the heat conductivity. Another important parameter is the quantum efficiency, which in the case of high Yb concentrations may decrease strongly due to energy migration between the Yb ions and finally deexcitation at certain impurities. To overcome this drawback either crystals of extreme purity have to be grown or crystals with structures have to be used, where for reduced energy migration the minimum Yb-Yb separation is quite large like in KY(WO4)(2) (KYW) and LaSC3(BO3)(4) (LSB). Both routes have been investigated and successfully applied in thin-disc lasers. (c) 2004 Elsevier B.V. All rights reserved.