This thesis reports the results of investigations into efficient, miniature solid state lasers and optical amplifiers in rare earth doped oxide crystals. Waveguides doped with the trivalent thulium ion were grown in YAG and YSO host crystals and lased, when optically pumped, at 2.012µm and 1.884µm, respectively. The Tm:YAG laser had absorbed power laser thresholds as low as 7mW and slope efficiencies as high as 68%. The planar devices were fabricated using the technique of liquid-phase epitaxy which produces waveguides of excellent optical quality and with very low propagation losses. High gain optical amplifiers have been demonstrated in Nd:YAG planar waveguides pumped by a Ti: sapphire laser and a diode laser. A small-signal gain of 28.4dB was obtained in a waveguide fabricated by liquid-phase epitaxy; in the same waveguide 290 mW of power was extracted for only ~550mW of absorbed pump power. A small-signal gain of 23.5dB was achieved in a Nd:YAG waveguide, fabricated by thermal bonding, for 250mW of absorbed pump power. Pumping directly into the upper laser manifold of a bulk Nd:YAG rod, at 869nm has produced a highly efficient laser on the quasi-three-level 946nm transition. The lower energy defect for a laser pumped in this manner relative to conventionally pumped Nd:YAG lasers should result in higher slope efficiencies and fewer thermal problems when pumped at high powers. Using such a pumping scheme a slope efficiency of 75% with respect to absorbed power was obtained.