The hydrothermal method is an excellent way to grow large, high quality single crystals for various applications. In this talk the method will be reviewed briefly with the emphasis on crystals for optical applications. The descriptive chemistry and growth efforts of two particular systems will be discussed in some detail. One system is the borates and berylloborates used for deep UV non-linear optical applications. The chemistry, growth and characterization of acentric borates with band edges below 200 nm will be discussed with particular focus on materials that can be used in second harmonic generation of UV photons.

The second class of materials is the lanthanide sesquioxides, Ln2O3, particularly Sc2O3 and Y2O3. These seemingly simply materials are exceptionally difficult to grow as single crystals because they are so refractory and melt near 2800K. They are of interest however because they are excellent hosts for solid state lasers and have outstanding properties for extremely high-energy applications. They are thermally stable, have good ligand field parameters and very high thermal conductivity. Despite these promising properties, the materials have been almost ignored because of the difficulty in handling them. We recently developed a very convenient method for the growth of these single crystals. This talk will focus on the growth techniques as well as the doping of these materials for laser applications. The spectroscopy of the doped materials will also be discussed briefly.