This chapter reviews the thermal conductivity (k) of the diborides of Zr and Hf. The evaluation is broken into discussions of the single phase borides, effects of additions, and finally the electron (ke) and phonon (kp) contributions to the total thermal conductivity (kt). Multiple factors subluence the conductivity of ceramics: that is, purity (second phases and solid solutions), polycrystalline effects (grain boundary resistances, and grain size), and density. Purity of the diborides is the most important factor affecting k. Comparison of previous studies revealed that the highest conductivities were achieved for the most pure materials, which were reaction processed from the metal or metal hydride and boron. Solid solution contaminants in the form of transition metals decrease k and can change the slope of k versus T from negative (for pure materials) to positive depending on the impurity and concentration. Second phase additions reduce k by acting as a lower conductivity phase or through secondary effects like producing microcracking. Research regarding ke and kp indicate the electronic portion is the largest contribution to kt.