Semiconductor clathrates have been suggested as promising material of high efficiency thermoelectric applications. The electronic structure and stability of the clathrates are critically reviewed. We show that resonant scattering of the metal ion with lattice phonon is the principal mechanism for the anomalous glass-like behaviour of the thermal conductivity. This unusual property exists as long as the interactions between the metal dopants and the network atoms are dominated by ionic interactions. We further show that semiconductor clathrates with fully occupied cavities are not ideal materials. Low occupancy is required to improve the Seebeck coefficients. A general scheme for the design of high efficiency thermoelectric materials is described.