In general, the ZT can be improved either by increasing the power factor or by decreasing the thermal conductivity. Increasing the power factor typically entails increasing the conductivity, which in turn increases the electronic thermal conductivity; since ZT is inversely proportional to the sum of the electronic and lattice thermal conductivities, this can effectively negate any improvements to the power factor. In this section, we therefore focus instead on the lattice thermal conductivity. We first establish the microscopic origin of the low lattice thermal conductivity in Y 2 Ti 2 O 5 S 2 by examining the contributions of individual phonon modes, and we then explore the effect of nanostructuring which, together with alloying, is a common approach to reducing the thermal conductivity and optimising ZT.