Scattering of phonons by defects and the consequent reduction in lattice thermal conductivity is reviewed, with emphasis on ordinary and high temperatures, and on point defects and extended defects. The reductions in thermal conductivity due to point defects and extended defects are approximately additive at these temperatures. Thermal conductivity measurements, both at low and at high temperatures, can yield information about the defects produced by radiation damage. Isolated point defects make an important contribution to the reduction in thermal conductivity at all but very low temperatures. The reduction in thermal conductivity of alumina is discussed. Another model of partial conversion to the amorphous state could apply to glass-forming solids such as quartz and zirconolite. At high temperatures one can treat a partially converted crystal as a two-phase system, with the thermal conductivity of the crystalline phase reduced by scattering at the interphase boundaries, and probably also by point defects.